One thing is gbc.fill = GridBagConstraints.HORIZONTAL; and the other is
modifying your weightx value like gbc.weightx = 1;.  Lots of things can
happen when you start playing with those constraint values.

I've still had no luck with it at all, I'm completely at a loss as to
why I can't get it to behave how I want!! If I post the relevant
sections of code I would be very grateful if some of you could have a
look at it and point out any errors to me. The problem is that before I
added the JTable, the GUI set up exactly how I wanted, according to the
grid bag constraints I specified (or so I believe anyhow), but now I
have added the JTable, the JTable panel is much wider than it needs to
be, leaving empty space to the right of the table and mucking up the
layout of the other components in the GUI,

Many thanks,
Steve

code....

class SlopeSketchPanel extends JPanel
{
    // declare variables
    public JPanel sketchArea;
    public JPanel dataArea;
    public JPanel keyArea;
    public JPanel scaleArea;
    public JLabel layerLabel;
    public JLabel layerNo;
    // declare instances of objects created
    public SlopeGraphicsPanel slp;
    public DataTable dataTable;
//    public ScalePanel sp;
   
    // constructor
    SlopeSketchPanel()
    {
        // create instance of SlopGraphicsPanel class
        slp = new SlopeGraphicsPanel();
        dataTable = new DataTable();
//        sp = new ScalePanel();
       
        GridBagConstraints gridBagConstraints = new GridBagConstraints();
        sketchArea = new JPanel();
        keyArea = new JPanel();
        dataArea = new JPanel();
        scaleArea = new JPanel();
        setLayout(new GridBagLayout());
       
        // create sketch area panel
        sketchArea.setLayout(new BorderLayout());
   
sketchArea.setBorder(BorderFactory.createCompoundBorder(BorderFactory.createEtchedBorder(),
BorderFactory.createEmptyBorder(15,15,15,15)));
        // add slope graphics panel
        sketchArea.add(slp, BorderLayout.CENTER);
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 0;
        gridBagConstraints.gridwidth = 7;
        gridBagConstraints.gridheight = 7;
        gridBagConstraints.fill = GridBagConstraints.BOTH;
        gridBagConstraints.weightx = 70.0;
        gridBagConstraints.weighty = 70.0;
        add(sketchArea, gridBagConstraints);
       
        // create scale area panel
        scaleArea.setLayout(new BorderLayout());
        scaleArea.setBorder(BorderFactory.createEtchedBorder());
        gridBagConstraints = new GridBagConstraints();
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 7;
        gridBagConstraints.gridwidth = 3;
        gridBagConstraints.gridheight = 3;
        gridBagConstraints.fill = GridBagConstraints.BOTH;
        gridBagConstraints.weightx = 30.0;
        gridBagConstraints.weighty = 30.0;
        add(scaleArea, gridBagConstraints);
       
        // create key area panel
        keyArea.setLayout(new FlowLayout());
        keyArea.setBorder(BorderFactory.createEtchedBorder());
        gridBagConstraints = new GridBagConstraints();
        gridBagConstraints.gridx = 3;
        gridBagConstraints.gridy = 7;
        gridBagConstraints.gridwidth = 4;
        gridBagConstraints.gridheight = 3;
        gridBagConstraints.fill = GridBagConstraints.BOTH;
        gridBagConstraints.weightx = 40.0;
        gridBagConstraints.weighty = 30.0;
        add(keyArea, gridBagConstraints);
       
        // create data display area panel
        dataArea.setLayout(new BorderLayout());
        dataArea.setBorder(BorderFactory.createEtchedBorder());
        // add data table
        dataArea.add(dataTable, BorderLayout.CENTER);
        gridBagConstraints = new GridBagConstraints();
        gridBagConstraints.gridx = 7;
        gridBagConstraints.gridy = 0;
        gridBagConstraints.gridwidth = 3;
        gridBagConstraints.gridheight = 10;
        gridBagConstraints.fill = GridBagConstraints.BOTH;
        gridBagConstraints.weightx = 30.0;
        gridBagConstraints.weighty = 70.0;
        add(dataArea, gridBagConstraints);
    }
}

class DataTable extends JPanel
{
    public DataTableModel tableModel;
   
    DataTable()
    {
        this.setLayout(new BorderLayout());
        tableModel = new DataTableModel();
        JTable table = new JTable(tableModel);
        TableColumn propertyColumn = table.getColumnModel().getColumn(0);
        DataTableCellRenderer cellRenderer = new DataTableCellRenderer();
        propertyColumn.setCellRenderer(cellRenderer);
        propertyColumn.setMinWidth(130);
        propertyColumn.setMaxWidth(130);
        propertyColumn.setPreferredWidth(130);
        TableColumn valueColumn = table.getColumnModel().getColumn(1);
        valueColumn.setMinWidth(175);
        valueColumn.setMaxWidth(175);
        valueColumn.setPreferredWidth(175);
        // set table preferences
        table.getTableHeader().setResizingAllowed(false);
        table.getTableHeader().setReorderingAllowed(false);
        table.setCellSelectionEnabled(false);
        JScrollPane tableScroll = new JScrollPane(table);
        add(tableScroll, BorderLayout.CENTER);
       
    }
}

class DataTableModel extends AbstractTableModel
{
    // create data model
    String[] columnNames = {"Property", "Value"};
    Class[] columnType = {String.class, String.class};
    Object[][] data = {{"Layer Number", "layerNumber"},
                        {"", ""},
                        {"Soil Parameters", ""},
                        {"Cu", "cu"},
                        {"Phi", "phi"},
                        {"Cohesion", "cohesion"},
                        {"Bulk density", "bulkDensity"},
                        {"Dry density", "dryDensity"},
                        {"Submerged density", "submergedDensity"},
                        {"Moisture content", "moistureContent"},
                        {"Liquid limit", "liquidLimit"},
                        {"Plastic limit", "plasticLimit"},
                        {"Notes", "notes"},
                        {"", ""},
                        {"Main Constituent", ""},
                        {"Type", "mainType"},
                        {"Name", "mainName"},
                        {"Strength", "mainStrength"},
                        {"Consistency", "mainConsistency"},
                        {"Grain size", "mainGrainSize"},
                        {"Main colour", "mainMainColour"},
                        {"Secondary colour", "mainSecondaryColour"},
                        {"Plasticity", "mainPlasticity"},
                        {"Comments", "mainComments"},
                        {"", ""},
                        {"Secondary Constituent", ""},
                        {"Type", "secondType"},
                        {"Name", "secondName"},
                        {"Strength", "secondStrength"},
                        {"Consistency", "secondConsistency"},
                        {"Grain size", "secondGrainSize"},
                        {"Main colour", "secondMainColour"},
                        {"Secondary colour", "secondSecondaryColour"},
                        {"Plasticity", "secondPlasticity"},
                        {"Comments", "secondComments"},
                        {"", ""},
                        {"Minor Constituent", ""},
                        {"Type", "minorType"},
                        {"Name", "minorName"},
                        {"Strength", "minorStrength"},
                        {"Consistency", "minorConsistency"},
                        {"Grain size", "minorGrainSize"},
                        {"Main colour", "minorMainColour"},
                        {"Secondary colour", "minorSecondaryColour"},
                        {"Plasticity", "minorPlasticity"},
                        {"Comments", "minorComments"}};
   
    public int getColumnCount()
    {
        return columnNames.length;
    }
   
    public int getRowCount()
    {
        return data.length;
    }
   
    public String getColumnName(int col)
    {
        return columnNames[col];
    }
   
    public Object getValueAt(int row, int col)
    {
        return data[row][col];
    }
   
    public Class getColumnClass(int c)
    {
        return columnType[c];
    }
   
    public boolean isCellEditable(int rowIndex, int vColIndex)
    {
        return false;
    }
   
    public void setValueAt(Object aValue, int r, int c)
    {
        data[r][c] = aValue;
    }
   
    public void updateData()
    {
        fireTableDataChanged();
    }
}

class DataTableCellRenderer extends JLabel implements TableCellRenderer
{
    DataTableCellRenderer()
    {
        setOpaque(true);
    }
   
    public Component getTableCellRendererComponent(JTable table, Object
value, boolean isSelected, boolean hasFocus, int row, int col)
    {
        setBackground(Color.lightGray);
        if (row == 0 || row == 2 || row == 14 || row == 25 || row == 36)
        {
            setFont(new Font("Dialog", Font.BOLD, 12));
            setText(value.toString());
        }
        else
        {
            setFont(new Font("Dialog", Font.PLAIN, 12));
            setText(value.toString());
        }
        return this;
    }
}