Thanks everyone for all the time and thought you've given to this.
A bit of background that I skipped in my original post:
After suffering a number of RCD trips last year (one of which tripped the kitchen MCB) I called in the qualified electrician who had wired a new ring in the kitchen when it was refitted several years earlier. He spent half a day with test gear checking everything. He found one possible fault, in the cable that ran from the last socket in the kitchen ring back to the CU. Since this cable was totally inaccesible, he just isolated it. This converted the kitchen from a ring to a radial and to match this he changed the MCB to 20 Amps. He left saying "you'll have no more trouble now".
3 weeks later the RCD tripped again but could be immediately reset.
At this point I thought I'd better check the appliances one at a time. So disconnected the oldest, rarely used fridge (not on the kitchen ring) and for good measure removed power from the DW.
Bad move in hindsight, to make two changes at the same time.
But as no more trips for 9 months, convinced myself it was the old fridge, possibly causing a problem when it went into defrost mode.
Then just recently, after I'd used the DW again and forgot to isolate it, we get RCB trip and MCB trip on the kitchen circuit.
So this is where my post started from, couple of days ago ...
For info, the DW is built in, so a real pain for ayone to look behind it. It's power cable comes out the back, reappears in the cupboard next to the DW and is hard wired to a thing (not sure what it's called!) with a switch and fuse - not plugged into a socket. I don't know if this 'thing' is on the ring or a spur off it.
I'm not an electrician but got a bit of common sense so the facts I'm grappling with are:
- intermittent
- always trips the RCD (there's only one in the CU)
- sometimes trips the kitchen MCB
- can always be reset - straight away if I happen to notice the trips
- no smoke
- all appliances still work
- seems most likely to be related to the DW since it stopped for the 9 months with DW isolated
Meanwhile the DW stays isolated and see if this stops the trips again. If so, get someone to check the DW and it's supply.
And BTW, the suggestion of getting an au pair has been clobbered by 'her' ...