Dnsmask doesn't do IPv6 very well, so I've installed ISC Kea, which feels a bit over-engineered, but so far so good.

I've got it installed and running with a tftp server defined in the config, but I've left it switched off for now because I need to check that it's not going to try to stomp on the BT router.

Poot. Just read the docs for Kea, and it won't run as a 'proxy' (that is, send out DHCP replies with options set but no ip address).

Bollocks to it, I'll get an OS installed on the USB stick, and then I can use that Pi for whatever.