Fault on Myson Apollo Fanfare30/50?

Hi,

My boiler packed in a month or so back, when switched on the fan would just make a struggling sort of noise ( sorry for the explanation ) Fitter checked it out and changed the fan for a reconditioned one. It was off for 2 weeks while being reapired. anyway he fitted the new fan and hey presto it worked straight away.
My proble is 2 weeks later it stopped working again, thinking it was a faulty reconditioned fan we paid for another one and had it installed but it is doing exactly the same thing.

Switch it on and the fan struggles to turn, turn it off and the fan whirrs away like a good un, switch it back on to fire and it groans and slows to almost nothing again. It's like as if the current isn't right getting to the fan when the switch is pressed to ON. Switch it off again and the fan spins up super fast instantly.

Any help is much appreciated guys, being xmas I am skint and it's getting costly.

thanks in advance

Martin