Resistance to earth: (dead test)
This test confirms the integrity (resistance) of the insulation between L and E on the boiler terminal strip or plug top pins (Black lead firstly to E pin and red lead secondly to L pin). Where the boiler supply is connected to a fused double pole switch, it should be isolated at the ring main supply end, to eliminate any possible, further faults present on the circuit (This is why a 3 pin plug is a much better option as a double pole spur can cause further problems).
With the boiler switches, time switches and thermostats on and the multimeter set on ohms scale, the resistance reading between L and E should read no other than O L ( infinity ) and if so, there is a fault .
A detailed continuity check is required to trace the faulty component before proceeding any further.