Lee wrote:

Only change made as a result of trying to fault find that issue* was to
replace the MCBs with RCBOs, of course that could well be the issue in
itself but it's been fine for the several months since then.

That way you've got 30mA of allowed leakage per circuit, rather than
30mA combined for half (or all of) your circuits.

the grill element eventually failed in a nice shower of sparks and
melted metal

There I was thinking finding my faulty oven element was a
boring/unsatisfying fix on the grounds it looked in good nick ...