Ged, deleting cookies was one of the first thing I tried, and sadly, hasn't improved things for me.
No surprise really, as I said I couldn't see anything in the cookies which seemed likely to affect it.
Good to see you're a Debian user too, as this was something that crossed my mind as being a possible cause for my problem. It still could be the problem as I have the disks next to me to upgrade to Squeeze and the newer version of Iceweasel. Just a case of finding the time to upgrade, but using an older version has made me wonder.
FWIW I'm using Iceweasel 3.5.16 (Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.16) Gecko/20111108 Iceweasel/3.5.16 (like Firefox/3.5.16)) which is as up to date as it gets on Debian Stable.
On Debian, if you don't want to do a full upgrade you can get a more recent version of a package using something like
apt-get -tsqueeze-backports install iceweasel
although it won't work for all packages because (a) not everything gets into backports and (b) there might be unmet dependencies. I haven't checked if you can get the latest Iceweasel for stable (Squeeze) in an oldstable (Lenny) installation. There were quite a few irritations in the older Iceweasel that I was using under Lenny, things like the BBC News site crashing it all the time, but I don't remember having any trouble such as you've described with the BCUK site.
You really should be doing a full dist-upgrade anyway, because security fixes for Lenny will cease in February.
Though I don't see why the VBulletin at NBC should work for me and not here.
There are vBulletin installations and vBulletin installations. I suspect that there may be some issues with the changes that have been made to the BCUK site recently but I have no real evidence.
The styles thing appears to be a possible cause for me as I can only choose between mobile and test, are these the only options that others see?
By 'styles thing' I take it you mean mean 'forum skin'? I see these options in the drop-down box:
Use Forum Default
Mobile Style
Test
AFAIK my setting has always been 'test'.