Ferroli Domina 80E Combi: No Heating or Hot Water?

Hi we have not heating or hot water, the fan starts up but the iginstion light does not come on We have had two different corgi registered men out to it first one said it was the PCB board, which we replaced at at cost of £180, however, when he tried to put the housing back on the iginition went out and it was back to square one, the other guy said it is the fan which is now on order at a cost of £150, can anyone give us advice on buying this fan, I don't want to spend anther £150 then £50 to fit the fan just for the same thing to happen when they put the housing back on. Would it be cheaper to purchase a new combi boiler? Or does anyone have an idea what the problem is? Thanks