It seems expensive to me.
The system will be drained and flushed which involves connecting a hose to the drain point and opening the taps and waiting a while - maybe half an hour in total.
The radiator valves can cost anything between £10 and £40 each and can be fitted in about 20 minutes each.
I don't know what re balancing radiatoes involves, perhaps just releasing the air in each one. If so, that's 10 minutes each.
Servicing the boiler could mean anything. If there's no noticeable fault, they will tap the side, look at the taps and dial and then leave it alone. Another 10 minutes.
A good digital room thermostat could cost £100 and, depending where it is, might take a couple of hours to fit.
I believe someone else has taken over Corgi's work but you can still use the web site www.trustcorgi.com to find other candidates for the job.