Overnight my DSL dropped and when it reconnected a minute later there was no IPv6 and latency had doubled.
Reconnects and router reboots, still the same. In the router log I can see I'm now connected to BNG3.WH-MAN-RE0 which is in Manchester whereas I used to connect to London gateways.
Phoned support and apparently some sort of regrade was done on my line recently - this may be the first time DSL has dropped since that was done. Waiting for support to get back to me.