Nearly all parts for these budget boilers cost an arm and a leg plus vat so don’t play spare part surgery, there’s several possible reasons why it’s locking out from a lazy fan thats dusted up and not clearing the products of combustion quick enough so the overheat stat is activated, the pump could be very weak, again causing overheating the heat exchanger could need cleaning if the fins are blocked or the overheat stat itself has degraded.. the only way forward is to get a gas safe registered engineer in. If you do need parts, make sure you’re sitting down when the cost is about to be given to you.
Phill Dee, March 2018