Finally, after many, many, moons, I've pulled my finger out of my behind and eventually (mostly) configured the Cisco 877W I picked up to replace the O2 Wireless Box II, which was provided free with my ADSL connection, and was overpriced, even at free. It frequently locked up, dropped the connection, and just wouldn't work with a VPN connection - I could connect, and "curl -I matstace.me.uk" would return the headers correctly, but "curl matstace.me.uk" sat and hung. For some reason, web traffic just wouldn't pass through.
The Cisco, although the sync speed is the same, and the speed test is the same, seems snappier. Probably just me imagining things though, good old "shiny new things" effect.
Once I've tidied up the configuration a bit, I'll post up a page on here with a working O2 ADSL DHCP config on it, as it's been reasonably tricky to come across.
And now that I've got my VPN connection sorted, I can finally watch Blackballed: The Bobby Dukes Story on Hulu this weekend.
If only I'd turned things like my NAS back onto DHCP, rather than leave it on the old network range.
And saving the running config I had which allowed me access via ssh/telnet might have been a good idea. I'll be balancing the netbook on top of the fridge tomorrow with the USB serial convertor hooked up to the console port on the router.