Now I'm confused, much happier and still annoyed. I logged back into Vista 32 on the Dell M1330 and it connected to the D-Link router quite happily. I'm certain I tested this combination of settings yesterday with no joy but perhaps there was some sort of patch stuck in the pipeline. Clearly I forgot Rule #1 (Give it a good kick, turn everything off and restart before trying to do anything).
Performance was pretty poor though - Ixia's QCheck gaves me results in the 600kbps -> 1.5Mbps range between the Desktop (connected over 802.11n) and the laptop (connected over 802.11g) which is extremely poor even for 802.11g/n compatibility mode. Intel had much more recent drivers than those supplied with the machine or up on Windows Update so I downloaded and installed them and now I have transfer rates in the 5-7Mbps range which I suspect is as good as it gets when you are actively mixing standards concurrently on the same router.
It's still pathetic that we can still have so many problems with the basics of Wireless Networking. someone really should sort that 802.11 crew out once and for all.