In uk.d-i-y, wrote:

4 The RCD may have got hyper sensitive and or may be at the
sensitive end of the allowable range. The only way to check is to
replace it really (though an electrician should have proper test
equipment to check it), fairly expensive again.

On the last point - it's not *that* expensive in my slightly dated
experience: 50quid 5 years back got me a wiring inspection by a very
competent local electrician, who spend time with his RCD-tester tracking
down the reason for nuisance tripping (combination of cooker elements
+ computers, as I'd suspected, along with an RCD right on the lower limit
of sensitivity: a "30mA" nominal out-of-balance trip rating means the
RCD must *not* trip at below half the rating, 15mA, and *must* trip
at-or-below the rating within something like 40ms; mine was tripping at
about 14mA.)

On the information you've now given, it does sound as if your installation
could benefit from a better allocation of circuits between the RCD and
non-RCD side, if not a dedicated RCBO or two. The immersion heater and
cooker circuits are prime candidates for moving to the non-RCD side, as
would be upstairs sockets (only likely to help with your nuisance trips if
you have interference-suppressed loads up there, such as computers). The
kitchen ring, if you have one that's separate from other downstairs sockets,
is also a fair candidate for non-RCD operation, but there's some conflict
between wanting to avoid nuisance trips from wash-mosh/dosh-wish, versus
the possibility that someone might run outdoor things from a kitchen
socket, and (back the other way) the loss of freezer/fridge contents if
the trip goes while you're away for two weeks ;-) One reasonable resolution
of the conflict is an RCBO dedicated to the kitchen circuit - this
combines the RCD and MCB functions in one unit, and allows the "full"
somewhere-between-15-and-30mA leakage "allowance" for just that circuit,
rather than across all the things on the RCD side of the CU. Costs of
RCBOs are falling - even a couple of years ago they ran at about 50quid
retail, now it's closer to 30-35; if a Regs tweak encourages their wider
use expect to see prices fall further.

Swapping circuits between the two sides can go from trivial to pain-in-the-bum, depending on whether there's enough slack in the cables reaching the
relevant positions, and whether the CU has room for repositioned MCBs and
RCBOs (which from most manufacturers take up two positions in the CU).
If you feel it's beyond you, I'm sure an electrician can be found who'll
do suitable Magick, at the possible cost of encouraging you to upgrade
the whole of the installation to latest Regs standard. You might get a
decent deal if you combine the 'can you tell me what's going wrong' with
the 'can you sort it out then' aspects ;-)

HTH - Stefek