I've just booked yet another engineer visit for Friday 14th.
This will be about the fourth visit in as many months for the same problem. Unfortunately, I cannot contact VM by email (for example) to ask them not to send an engineer without knowledge of the history of this.
The last engineer said that an external network engineer would have to run new cables, so I don't want an 'internal' engineer coming.
The engineer who came on 14th (who had been given no info on the history of this fault!) said that he thought the cable entering my house was faulty, and that a network engineer would need to replace it. I've heard nothing since.
The complaint that I logged online has not been responded to.