follow the thermostat wire it leads to a little brown box
it should just pull off the boiler its just by the temp dial. and on the block. there are a code of numbers you can copy down or you can take the whole thing to a plumbers merchants i got one my self a couple of years a go and it only cost about thirty pounds.
by doing it your self think of how much you will save.
p.s if you decide to do it your self dont forget to turn off the gas to the boiler and the electric.
hope this helps you
tony
March 2010