Well, I'm answering my own question to some extent - I have bought a Draytek 2860 and have managed to get it set up using its built-in VDSL2 modem with the static IP addresses, NAT'd internal LAN, etc but can't get an IPv6 IP address even after following the Draytek documentation for this. The IPv6 side keeps retrying the PPP connection so that after about 16 to 18 seconds the connection drops and it retries once again.
It's possible IPv6 isn't yet available on my local BT exchange - can anyone confirm whether the Crouch End, London N8 exchange with phone numbers beginning with 020 8341 is actually IPv6 enabled?
Andy