Hmmmm………….. How much have you got? LoL
First you can’t have thermostatic radiator valves on all of your radiators, you need to have one has a by-pass valve, this is normally the radiator in the bathroom, it’s always on.
You need this to stop your boiler from over heating, but you could also use a towel rail in the kitchen or fit a towel rail in the bathroom to do the same job.
Its about £200-300 for power flushing but if you have it done with the new valves, it should be cheaper. The thermostatic radiator valves start at about £10.
But all the work should be done in a day and we would charge about £400 for the work.
Personally I think power flushing an old heating system is asking for trouble, if any of the old radiators are rusty in side, they all leak after and you may have to replace them.
Remember the labour cost is 90% of the price.
All ways get 3 quotes and use your local trading standards web site to find a plumber or the “ihpe “web site
http://www.iphe.org.uk/index.html