leaking radiator valve on a sealed system

[HarrogateGas]

Fact that it's a sealed system makes the job no more difficult -
in fact refilling after draining down is quicker.

Maybe you envisaged doing it without draining down. Unfortunately this isn't really an option with a sealed system. You must drain down (or freeze pipes). If your plumbing skills are not the best I would advise draining down in all cases as you are more likely than not to get a leak after doing the repair/ valve swap.

B*llocks!


Have to drain a sealed system is news to me.

Surely just drop the pressure, preferabley at a point below where your working and close auto air vents. The system will then hold it water.