John Rumm brought next idea :
Homeplug devices work by injecting a RF signal into the mains wiring. Given
that was never really designed to limit radiation at those frequencies, some
of it will escape[1] - and may in theory make its way into the telephone
lines where it could interfere with the RF carrying the broadband. In
practice I have not seen this as a problem.

Yes, that was also my best guess at the cause and what Plusnet seemed
to be suggesting. Possible it is also a faulty or poor quality modem
router, hence all of his wifi range problems.