Yes, I found that too, Richard. I checked my version of URLMON.DLL and it's later than theirs, as you'd expect with SP2 applied, so I didn't go for the hotfix.
Odd that one machine is ok and the other isn't, though the software environment is virtually the same.
Thanks for sticking with me on this, but I think I'll give up on it for now - I have a couple of workarounds, so all is not lost. Perhaps it'll fix itself in some unannounced way - it's happened before.
Cheers
AA