Potterton 60E tripping electrics on startup?

I have a Potterton Profile Netahaeat 60E which seems to trip the RCCD switch on my eletrics every time it tries to come on. It doesn't even start the pilot light before tripping so I'm wondering if this is a fan problem or a stuck Honeywell switch, or posssible pump problem? A few days before this happened there was a loud whirring noise coming rom the immersion cupboard where the pump and honeywell switches are positioned. Has anyone out there had this problem, and if so, how do you fix it without buying everything and trying it out? I have replaced the pressure switch, gas valve(via corgi plumber) and PCB in the past from allspares direct for £30 each time, but their fan alone os £55 plus postage, so I don't want to waste money on things I may not need to get things going!