Thanks Alessandro,

To update, it seems like this is only an issue with IE. This version of Sophos installs an add-on, Sophos Web Content scanner, in IE. By turning this off (as a test) we were able to get a streaming connection again.

We're using SSL, but given that Smart Polling seems to work well we've got some time to work out different ways of configuring Sophos.

An alternative workaround is to use IE8's In-Private mode - this appears to disable add-ons in case they might record history data. The Sophos plugin doesn't run in this mode :-)

Cheers,

Richard