Can anyone identify this pressure valve or at least its connector type?

Question

I recently moved into a house and heard a dripping noise coming from the airing cupboard. Worked out that a pressure relief valve was letting water through and had a plumber come out to look. He said the expansion tank and relief valve we'd to be replaced, the only snag is that having taken photos of the parts and gone away, he's been unable to source the valve because of the fitting it has, which appears to be a bayonet or similar. I'm going to contact the builders of the house to see if they can help out, but can anyone identify this part or tell me the name of that kind of connection? Haven't had any luck searching so far but I know little about plumbing yet.

The got water tank is an Ideal Thermstore and Ideal have similar looking 1.5bar valves, but all the ones I've seen have threads for both connections.

I'm in England.

Answer 1

As Steve Wellens suggested, it's a Caleffi valve that uses a circlip attachment. The reducing and relief valve are sold with the body as a single unit (called a multibloc) and that connection seems to be particular to that. Did manage to find a new relief valve separately once I knew it was a Caleffi item.