Well, a few hours after that post about changing wifi channel numbers and I have learned a little more than I expected about WiFi configuration. Turns out that changing the channel number made a big difference to a particular machine on my network, my desktop PC.
The other machines were happy following the wifi around the different channels, but my Dell 5000 (with exactly the same card and drivers as the old Dell 4100) would only work on Channels 1,7,8,9,10,11,12 and 13. As you can guess I was working my way through the channels in sequential order and that big gap at the top lead to all sorts of changes by me to try and trace the problem.
Of course the engineer side of me should have said “The only thing that has changed is the channel”, but I was in there rebooting routers and fixing wep keys. Then I discovered a post about channel powers and worked through the options – that was the issue.
But what was really confusing was that everything else was happy reconnecting, just not the machine I was using. Strange. So I’m sticking to channel 7, which has the best connection speed.