@burakkucat - quite so.
I do need to get a separate heavy earth connection in the office for general duties of this kind.
Just a tiny improvement would gain me a 3 * 0.25 Mbps d/s speed increase, as I am close to the 2272 sync rung on each one of the modems. One was indeed syncing briefly at 2400 earlier this week then back to 2240 which it is now holding at the usual target SNRM of 6dB and no errors reported. The biscuit tin / microwave oven thing is just for the sake of science, to see if any improvement is there to be gained.