Thanks Neil.

The correct site is:
http://p4eclipse.sourceforge.net/beta/

(Note: don't miss the final slash. It wouldn't work with Eclipse Update
Manager if missing)

The Perforce Team Provider is packaged as an Eclipse Feature, and is
designed to be installed from within Eclipse, using the Update Manager.
There is no way to directly download the plugins. Instead, you must
register this URL as an Eclipse Update site.

If you're behind an HTTP proxy/firewall, you need to invoke Eclipse
with the additional command line arguments so that the Update Manager
can use the proxy. If you're proxy is an authenticating proxy you will
be prompted for your userid and password.

Command line arguments for HTTP Proxies
eclipse -vmargs -DproxySet=true -DproxyHost=<proxy_name>
-DproxyPort=<proxy_port>

Available Update Sites
Official:
Release URL: http://p4eclipse.sourceoforge.net/site/
Description: Official releases are available here. Many different
versions are available.

Beta:
Release URL: http://p4eclipse.sourceoforge.net/beta/
Description: This holds the latest (and only the latest) un-official
release available. This version should always work, but it doesn't go
through any extensive testing. New releases tend to be made every
couple of feature/bug fixes. New beta release do not necessarily get
announced.