Well, the "CAT7" screened ethernet cable arrived thanks to eBay etc. It looks much better than the previous one, and it has nice metal shields over the plugs etc.
What you see here is
On the left, the Netgear router (it is just held up with Velcro, hence not vertical yet)
On the right the BT Openreach modem
Between them the thick blue ethernet cable.
The cable tends to try to push the modem over, as it is only 600mm long. Shorter cable, less chance of radiation I reckon.
A shelf might help, as Velcro on the wall is not ideal ...
So that seems to have killed the noise on the bands I use.
Both units come with "wall wart" switching PSUs which are still suspect. More work to do there with ferrites etc. I may replace them with a linear power supply (they are both 12 volt).
So far, I have had no other noise problems with the fibre broadband. It does not seem markedly faster but I have still to do a speed check.
As those screened CAT7 cables seem so much better for EMC purposes I am pondering over buying another couple and doing my neighbour a "free upgrade" for her BT system. It would cost a few £££ but it might knock down the noise level in here.