Now this is strange...
I disable the compression, and then I started getting the error every time on FF and IE8. Compression turned back on, and it's all working again.
Some support sites suggested that if the server is compressing by default, and you try to compress again, it may occasionally make a browser baulk, but this does not seem to be the case on the ICB server.
However, it is possible that because compression is enabled on the front end and the forum, that somehow this is crossing over, even though technically they are separate sites.
I have now disabled compression on the site front end and left it enabled on the forum, as this is the part that gets the most traffic.
Please have a test again and let me know if this has resolved it.
PS, to give you an idea of how much bandwidth it's saving, here's the page we're looking at now, before and after compression.
Size, Markup (bytes) 25,103
Size, Compressed (bytes) 5,208
Compression % 79.3