I've just come back to this topic and realised the story is incomplete.

I went back to AAISP to say I'd been told there was only Marconi in my exchange. They said no it isn't so I went back to Plusnet. They eventually managed to get me shifted to the alternative equipment and back on Max Premium - no more disconnects when the phone rang (and the strange gap between the upstream and downstream tones disappeared).

Plusnet 1/10
AAISP 10/10

Any chance of an honourable mention too ?

I had the answers the day after your first post.