Estimated boiler repair costs in E20 range from the cheapest Boiler repair £70 (RLUK & Co.Ltd based in IG3) to the dearest Boiler repair £220 (Kmgengineers based in RM6). 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 E20 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 E20

"Boiler repair pressure release valve replacement"
IG11
"Boiler repair Diverted value "
IG3