My Ariston Combi Boiler has tripped the electrics & will not turn back on -?

My 2 year old Ariston Combination Gas boiler has tripped the electrics. I have manged to get electric back on but cannot switch power back onto the boiler without all electrics tripping. This happened 4 months ago & at great expense had a valve replaced. It has now happened again & am concerned that the £500 repair we had done has happened again. I have called the same company again who assured me if it is the same problem work will be done free of charge. How will I know if they try and dupe me into paying all over again for the same part?