Leaking valve in Central Heating

[Fil]

I have a leak in a t-valve which lies between the hot water out and the return pipe to the combi boiler. The leak is not serious just a constant drip whilst the heating is on and seems to be coming through a join in the casing in the end not connected to the pipes.

Is there anything I can use to repair this small hole or should i really be looking at calling out the plumber and replacing this part?

I'm not sure what the valve is there for so if someone could enlighten that would be good too.

Cheers

 

[breezer]

without knowing exactly what it is it is hard to say, but is it one of these? or maybe bigger?

 

[Fil]

much bigger. At the end of the long section of the T is a multi turn 'turn thing' (sorry don't know what the technical term is). This seems to open and shut the flow of water as you would expect so that when open the central heating system is short circuited- quite why you would want to do this is what I don't understand.

I've had a closer inspection and it would appear the whole 'mechanism' screws into the casing and this is where the leak is coming from so I think it may be just a case of finding the right spanner and tightening things up.