Well.... perseverance paid off!
Replaced both MCB's, the 16A was very corroded. Opened up all fittings, all good but still shorting somewhere. Put my meter on continuity and there's a short across live and earth. Disconnected all fittings, tested each run of cable. Eventually found the problem cable, physically traced what I could get to and there's a split in the insulation and scorching :-).
Cut the cable, put a connector in and sorted. Put everything back together and... tripped again aaarrgghhhh.
Decided there must be another meltdown on that cable, bought new cable... Boom fixed... frustrating job but hey electricians are selective so no choice.
BUT.... having previously removed and Sikaflexed two windows, we turned up today and find 2" water on the cabin floor, devastated as we'd gone through trouble and expense of lifting boat, moving a Hull fitting and gel coating some crazing.
I'd sealed the water heater flue too as that had leaked, still leaking but the rain on the Lancaster had been torrential and blowing sideways I'm told.
You know the feeling of losing the will to live? Seems no matter what, each visit consists of pumping water and not cruising. And there's me thinking cruisers we're less agro than narrowboats 😞