Estimated boiler repair costs in E1 range from the cheapest Boiler repair £100 (Touch of Heat based in E1) to the dearest Boiler repair £220 (Boiler Repairs London based in E9). 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 E1 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 E1

"Boiler repair - replace pressure relief valve. We have already had the expansion vessel replaced."
SE16