Oh the joyous fun of networking. I tried the bluetooth bridge as suggested by IcelandDreams, but windows wouldn't allow a bridge with the Dell-WIDCOMM stack I'd installed. I went back to the old stack, but then whenever the bridge was created the computer strangely thought the LAN cable was disconnected.

Took speculatrix's advice and went back to creating an ICS'd Ad-hoc wifi network. Despite the n810 being able to see and connect to the wifi, it wasn't receiving an IP, gateway or DNS from the DHCP server.

Finally a breakthrough when I disabled encryption, and created an open ad-hoc network between the two. I thought I'd finally got the IP/DNS settings correct enough to try encryption, but WEP and WPA caused the same issue - the n810 connects to a Link-local IP address, and vista claims that the settings for the wireless network are incorrect. Deleting the network from the Network List, then reading doesn't work, as it claims the password used earlier is now wrong - I guess this is n810 inability to authenticate the network for other devices. After playing around with the settings I was able to get it back onto an unencrypted network.

All in all I'm happy enough to leave it as is, but if I get game later I'll try encryption again - enjoying the net on the n810 to much for the time being.

Thanks everyone for your help!