The day finally arrived. The broadband switch went through around 7AM, so I spent five minutes reconfiguring the router once I got up, and it's all working perfectly.
The port of my landline number to VoIP was a little less smooth - it spent most of Friday "in limbo" with calls neither ringing the handsets at home nor landing on VoIP - but by 6pm all was well.
So far the whole service Just Works including the IPv6. Result!
There's one more post to follow on what I'm doing to keep my old landline handsets working.