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I doubt it was the routers, I had my usual one, my spare one, and at different times I borrowed my woman's Thomson one, and also her BT Smart Hub one, and they all had the same issues. Now they have changed the encoding the ADSL ones won't connect, but her Smart Hub and my new Smart Hub will, and the problems appear to be gone. Based on hers having the same issue as all the others on ADSL and not on VDSL I have to conclude the issue wasn't with the routers but fixed by the new connection.
Coding / Signalling, you may well be technically correct, but it all means much the same to me. Either a box can interpret the data in the form it arrives, or it can't. Trouble is whilst I've had two lots of £129.99 demanded of me, I'm convinced is was not for having an issue on my premises but because they weren't competent enough to find it on theirs. There's a lot I can do with £259.98, especially on a pension. If they'd really found something on my premises, which I don't believe, then surely the problem would have been fixed first time and the second (third in reality) visit wouldn't have been necessary.