Hi Simon,
If you remove the iRule and make a request, do you get a response back from the VIP?
Does the application use absolute references in the response data (as opposed to HTTP response headers) to the domain you want to proxy (ihstestweb.xxx.xx:1234)? If so, you'll want to add a stream profile to the iRule and set the global variable ::rewrite_response_payload to 1. You should also add a custom HTTP profile with response chunking set to Rechunk.
If the app uses local references in links, etc, in the response payload, you don't need to add a stream profile to the VIP and you should set ::rewrite_response_payload to 0.
If that's not the issue then can you use Fiddler for IE or HttpFox for FF and see if you get any response headers or content back from the VIP?
Aaron