Any heating engineers here? What’s this part?

[User112]

It’s in a cupboard upstairs along with some piping and electric things (presumably the set up for our immersion heater). This is leaking, can anyone tell me what it is pls? Any idea how much it’ll cost to replace it?

This:

It' the motorised valve that controls the flow of water from the boiler to the hot water cylinder.

It' most likely leaking from the spindle of the brass body though worth checking that it's not leaking from either of the nuts on either end. Cost of the whole valve is from about £60.00 or yoj may ppick up the brass body alone on ebay for £10.00 if you can identify the make. It's an hour to change.