I have this valve which we use on the boiler to increase the pressure but it has just started leaking and is dripping through our ceiling.
Can anyone suggest what I need to do to stop the leak?
Would need to know more about the design of the system. I'm assuming I'm seeing plastic piping which leads me to believe that this is a fresh water take-off from a heating system (boiler). Plastic and metal do not live well together, and any fittings should be of the same type. Some may argue that the Shark-Bite and related connectors work well, and they do in certain situation for which they were designed.
I would suggest replacing all the plastic pipe with copper coming from the boiler until you get to the hot water house piping. Then the copper-to-plastic (CPVC) fitting(s) should be manufactured for that specific transition. The heating and cooling of the two materials expand and contract at different rates and is most likely the cause of the leak, as the plastic has probably deformed at the point of clamping by the valve.