Can anyone suggest what is going on? Could such an apparently
simple change to 30 yards of supply wiring make the trips more
sensitive? Is there any mistake they could have made?

For ref; The RCCB in the Consumer Unit is an MK LN5780, 240V
80A, with 30mA tripping current. Between the incoming fuse and
the CU is the old original trip, a Chilton "Voltage operated
Earth leakage trip, 60A Type D". The MK 30mA always trips,
and sometimes (maybe 1 in 5 times) the Chilton also trips.

Maybe the electric company did a megga test, and fried your RCD!

I have the same model MK breaker, and have had to replace it recently, not
due to a megga test, just seemed to have died, it was about 20 years old! -
as soon as we got the new one (same model) the annoying trips stopped!

One thing though, they aint cheap!

Sparks...