I think they are around £40

This will be the easyest way to do it Rob

Turn the main water supply into tthe house off.

Drain the water out of the heating system by putting a hose on a drain plug that will be on thedownstairs Radiator valve or a Brass valve on the downstairs heating pipes.
Once the water is running out the hose, undo the vent plugs at the top of the radiators upstairs, this will get rid of the water in the heating system.

Once the water has stopped running out the hose it is empty and you can remove the Zone Valve, BUT keep a towel and bucket close by as there may be a little water still left in it.

Once all the new valve is fitted, turn the vent plugs on the radiators off, do the drain plug up and refill the heating by turning the water back on

Good luck and God Bless

Let me know how you get on Rob