Estimated boiler repair costs in SE26 range from the cheapest Boiler repair £70 (Nat gas based in BR3) to the dearest Boiler repair £200 (P Hennessy based in SE25). This is an average cost, replacing a heat exchanger will be more expensive, a diverter valve less so.
Comparing gas heating engineer estimates for boiler repairs in SE26 gives you an idea of who will be better value but the best way to compare costs is to get actual quotes.
What to do - If your boiler is playing up, quickly get rates and quotes for boiler repairs by clicking "Get quotes" to post your job to heating engineers in SE26

"Increase the pressure on the boiler and service the boiler."
SE21