To answer some of the questions.

Both RCDs are 30mA.

The 16 circuits have:

4 ring mains
2 power spurs (cooker, immersion heater)
The rest are lighting circuits as far as I know.

The only connection we can think between the consumer units is the aerial on
the TV - we have TVs on both parts of the house. We tried disconnecting this
but we still got a trip. The aerial has a masthead amplifier powered by a
transformer on a normal socket fed by the main fused consumer unit.

As Dave says both consumer units connect to an earth which I think is
connected to the neutral in the incoming supply. There is a cable from there
to ground on the Manweb side of our supply about 30m from the house.

Hope that helps.

Thanks for all the suggestions.