Sorry about the delay in replying. I managed to sort my problem out,
but the SOAPAction header was a bit of a "red herring". I finally
managed to manipulte the content-length header as I needed to remove
the contents of the 200 OK status response. All works fine now. Just
to explain, I use the Reverse-via SOAP header to see whether the
request was from an external source, then I simply remove the body and
set the content-length to 0. The code is as follows:

package com.webswell.filter;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

class KillGhostFilterStream extends ServletOutputStream {
    private OutputStream intStream;
    private ByteArrayOutputStream baStream;
    private boolean closed = false;

    private String text1;
    private String text2;

    public KillGhostFilterStream(OutputStream outStream, String t1,
String t2)
    {
        intStream = outStream;
        baStream = new ByteArrayOutputStream();
        text1 = t1;
        text2 = t2;
    }

   public void write(int i) throws java.io.IOException {
        baStream.write(i);
    }

   public void close() throws java.io.IOException {
        if (!closed) {
            processStream();
            intStream.close();
            closed = true;
        }
    }

   public void flush() throws java.io.IOException
   {
        if (baStream.size() != 0) {
            if (! closed) {
                processStream();              // need to synchronize the flush!
                baStream = new ByteArrayOutputStream();
            }
        }
    }

   public void processStream() throws java.io.IOException
   {
        intStream.write(replaceContent(baStream.toByteArray()));
        intStream.flush();
    }

    public byte []  replaceContent(byte [] inBytes)
    {
        String retVal ="";
        String firstPart="";
        String tempFind="";

        String origString = new String(inBytes);
        String srchString = (new String(inBytes)).toLowerCase();

        if ((srchString.indexOf(text1.toLowerCase()) > -1) &&
(srchString.indexOf(text2.toLowerCase()) > -1)) {
            System.out.println("I found an empty Header and an empty body!");
            return "".getBytes();
        }

       return origString.getBytes();
   }

}
class KillGhostFilterWrapper extends HttpServletResponseWrapper
{
    private PrintWriter tpWriter;
    private KillGhostFilterStream tpStream;

    public KillGhostFilterWrapper(ServletResponse inResp, String text1,
String text2) throws java.io.IOException
    {
        super((HttpServletResponse) inResp);
        tpStream = new KillGhostFilterStream(inResp.getOutputStream(),
text1, text2);
        tpWriter = new PrintWriter(tpStream);
   }

   public ServletOutputStream getOutputStream() throws
java.io.IOException
   {
       this.setContentLength(0);
        return tpStream;
    }
    public PrintWriter getWriter() throws java.io.IOException
    {
        return tpWriter;
    }
}

public final class KillGhostFilter implements Filter
{
    private FilterConfig filterConfig = null;

    private String searchText1 = "<SOAP-ENV:Header/>";
    private String searchText2 = "<SOAP-ENV:Body/>";

    public void doFilter(ServletRequest request, ServletResponse
response, FilterChain chain)
        throws IOException, ServletException {

        java.util.Date date = new java.util.Date();
        HttpServletRequest req = (HttpServletRequest) request;
        String rev = req.getHeader("Reverse-via");
        if ((rev!=null) )
        {

            System.out.println(date+": "+req.getHeader("Reverse-via"));
            System.out.println(date+": "+req.getRemoteAddr());
            HttpServletResponse resp = (HttpServletResponse)response;
            KillGhostFilterWrapper myWrappedResp = new
KillGhostFilterWrapper(resp, searchText1, searchText2);
            myWrappedResp.setContentLength(0);
            myWrappedResp.setHeader("SOAPAction","\"ebXML\"");
            chain.doFilter(request,  myWrappedResp);

        }
        else
        {
            chain.doFilter(request,  response);

            //HttpServletResponse resp = (HttpServletResponse)resp;
            //resp.setContentLength(0);
            //KillGhostFilterWrapper myWrappedResp = new
KillGhostFilterWrapper(resp, searchText1, searchText2);
            //chain.doFilter(request,  myWrappedResp);
            //myWrappedResp.setContentLength(0) ;

            //myWrappedResp.getOutputStream().close();
        }

    }

    public void destroy()
    {
    }

    public void init(FilterConfig filterConfig)
    {
        this.filterConfig = filterConfig;
   }
}