Range from the cheapest Fit a radiator £120 (Touch of Heat based in E1) to the dearest Fit a radiator £240 (Boiler Repairs London based in E9).
Comparing heating engineers estimates for fitting a new radiator in Monument gives you an idea of who will be better value, but the best way to compare costs is to get actual quotes. Radiators vary in price, and additional hours will be required to make pipework changes.
What to do - Get rates and quotes for removing your old radiator and replacing it with a new radiator by posting your job to heating engineers in Monument.

"1. Fit two new radiators (note: piping has already been altered, so it's just installation) 2. Fit a new smart programmer (Tado) in place of an old one "
CITY OF LONDON
"Change one radiator to install a new own in place"
SE1