I have a 30ma earth leakage trip that is tripping every time a
> significant load (about 3KW) is applied to the "downstairs" ring main.
> The immersion heater will always trip it, the dishwasher in heat mode
> will usually trip it and I have seen daft things like the fridge trip
> it. Note that if the dishwasher is plugged into a different circuit
> but with all other appliances remaining 'as-is' the device won't trip.
> This doesn't make any sense to me as the trip is seeing the same load
> (and fault) regardless of which circuit is carrying the load.
It's earth leakage from the heating elements in the immersion and or dishwasher - fairly common, I'm afraid. The fridge may have a small amount due to RF interference suppression, and might just top up the amount needed to trip the ELCB.
Changing to an RCCD might well cure the problem, or could you re-wire the immersion to a non protected circuit - there's no need for it to be so protected, IMHO.