DIY Doctor UK DIY Forums

Got home today and the valve in the photo is leaking. Can someone tell me what it is called and what would cause it to leak constantly?please.
Is there a way to control the leak?

Thanks Matt

It looks as though it could be part of the expansion system, is it a bit like an open funnel where your finger is and does the lower part go outside? At a very vague guess could it be frozen outside and backing up?