Thanks all, understood. Whilst I understand the theory of draining the system and replacing this valve, with a newborn in the house during winter I can't risk a cock up, so professional it is.
Greatly appreciate the quick responses.
I have found a small leak from a valve on the cold water inlet to my boiler. It is on the boiler side of the main shut off valve for the filling inlet.
I assumed at first glance it was an isolation valve but would have expected a different type of head on it (slotted rather than hex bolt). Is...