Citrix clients not connecting

[Sheridan]

I've got a few (all Windows 7 so far) clients that can't connect to our Presentation Server 4.5 box (using an access gateway). Lots of other machines are working fine (Windows 7, XP,Mac, Linux using various browsers) but not these ones!

They login ok and display the applications, but trying to start any application brings a long delay ('Starting...') and eventually the message:

'The Citrix SSL Server you have selected can not be reached'

SSL on this is a proper certificate and not self signed, and as I said it working on many other devices!

Any ideas? Citrix themselves don't have any information for this error at all.

Edit: Just tried google chrome as well - same problem. I guess this will have to be put down to another one of those W7 things that don't work on certain machines!