i would go for the boiler i have had an earth fault quite a few times on the pump that can cause the trip to go, not enough of a fault to blow a fuse but with an rcd protecting the circuit it will trip it every time.
disconnect for the downstairs circuit replace the power to the boiler with a long lead and plug it in upstairs or
take the heating circuit to its own trip on the board and alter the type of rcd ie: try an Type S for a slower trip and still keep the 30mA required trip.
bit of info here that might draw light on the subject for you.
or you could find the fault and fix it .