Installed Windows XP SP2 on another harddrive to see if a clean installation would help. I also swapped the antenna to F plug adaptor for another one.
Now DTV can actually use the Twinhan DVB-C card, and it also finds all the channels! Success! I can't see them or listen to the radio channels, but I assume that's because I haven't actually installed any video codecs :)
I found a nice guide to using MultiDec plugins in GB-PVR
with a smartmouse/phoenix cardreader so I can use my subscription card from YouSee. The guide is here: http://www.hyperlinkx.com/pmwiki/pmwiki.php?n=Plugins.MultiDec
The next step is to get GB-PVR to find the channels and use the MultiDec plugins and the cardreader to descramble the channels (Viaccess).