I would repipe completely ,I am old school and prefer copper but if doing yourself speed fit is a easy option,have new rads correctly sized for running at a lower temperature to keep your boiler in condensing mode as much as possible.
The valve body can pass, incorrectly piped ( has it just started doing this)have you a gate valve on the heating circuit after the valve? If so shut it and see if it still gets warm.
Turn cold supply off will it will stop hot after a small amount has run off,no need for a hot isolation valve.
On another note it’s a very poor installation that is not to standard.