Just to let everyone know that if you're having trouble getting your mits on your welcome pack after you arrive (like we did), I managed to get ours in the end.
After filling in the card after we arrived and posting it, and after three weeks nothing and then a phone call to immigration (who claimed it came from the Citizens Advice Bureau, who confirmed it was Immigration), and another call to Immigration who'd said they'd send another copy out, I emailed them from their website after another three weeks. It turned up after three days! And the best bit is the road atlas.
Anyway, the link is here for the webform.
Good luck to those who haven't received theirs.