How can I get a reasonably priced Potterton Suprema Lume?

I have had my boiler for about 10 years I has not caused me much problem before,but for the past few weeks even after having to plumbers out it keeps cutting out. One came got it restarted and told me there's a problem with the restart button,he got it going again then it stopped a few days later called someout another time and was told the same thing. I've been quoted £386 for the part and to fit it,which is a huge amount for me at the moment. One plumber told me that these boilers Potterton superima are known to have this fault and I might get the part for free. Sorry to go on but can you help?