Estimated boiler repair costs in Outer London range from the cheapest Boiler repair £70 (Gas Install based in E16) to the dearest Boiler repair £200 (M K Heating based in E1). 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 Outer London 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 Outer London

"Boiler repair - replace pressure relief valve. We have already had the expansion vessel replaced."
SE16
"Boiler repair, replacing pressure relief valve"
SE8