Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
THE BOILER MAY NOT HAVE AN EMC FILTER DEPENDING ON ITS SERIAL NO. IF IT HASN'T THEN PROCEED AS IF CONTINUITY IS OK
(IE SHORT CIRCUIT MEASURED)
Remove the live and neutral leads from the EMC filter

Measurement:

Check for continuity across;
Test 1) The live in and out.
Test 2) The neutral in and out.
Replace wires after test.
Expect short circuit on both.