So some people on Plusnet appear to have some sort of IPv6 connectivity, others don't. I wouldn't know if Plusnet themselves have done anything because I'm using an ancient router which doesn't support IPv6.
Any chance certain routers have built-in IPv6 tunnelling? Also, I thought newer versions of Windows have built-in IPv6 tunnelling technologies, but I don't know if any are automatically enabled so that they just work.