RCD Tripping Poser (long)

I have a workshop a distance away from the house (approximately 60m). It is fed from an MCB in a consumer unit (with an RCD as a main breaker) through two x 2.5mm FTE. At the workshop it enters through another RCD then into a consumer unit with three MCB, two at 20A feeding two workshops and one at 6A feeding all lights. This consumer unit also feeds the underfloor heating and the immersion heater.
There is a second consumer unit that does the rest of the house - lighting circuits (protected by a single RCD) and upstairs and downstairs sockets on RCBOs and cooker, shower etc., also covered by an RCD. The incoming mains (pole route) is also protected through and RCD.
The reason for the RCBOs is that for a while, from what we believed to be an external influence, the "whole house" RCD would trip and we would lose everything. Now we have RCBOs protecting some circuits, so the losses are contained.
That's the background!
I am currently getting "nuisance" trips of the workshop RCD, although sometimes it also trips out unrelated RCBOs on the upstairs and downstairs sockets (a different consumer unit to the workshop supply in the house). At other times, just the RCD at the workshop trips.
I am trying to work out where to look. I have turned the workshop off totally at night (through the workshop RCD) and the house hasn't tripped. Not conclusive, but a start. I have removed circuits one by one from their MCBs and neutral bars (stopping the neural/earth fault as well) and this has shown that in two cases tried so far, disconnecting these circuits, leaving the others connected, doesn't stop the trips. I have two more circuits to work through (thank goodness it is cold as the freezer is on one of them!)
I haven't meggered the cables between the house and workshop yet (next weekend job when it is light) but my question is, could a fault in the house affect the "downstream" RCD in the workshop and how can the workshop RCD tripping affect "upstream" RCDs and RCBOs?
In other words, am I looking in the right place at the workshop end, or could I have a house fault reflected into the workshop RCD and I'm just wasting my time in the workshop?
Update - last night, with the workshop RCD off, I had the downstairs sockets RCBO trip out - am I chasing two independent faults? Must megger that cable between the two!
Confused of Wiltshire!
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