Ariston dia 20 leaking water from safety valve

[PSABIN8951]

Hi all, newbie to boiler servicing here so please forgive any stupid questions but I have an Ariston DIA 20 Combi which has an annoying problem which has recently started. With pressure set between 1 & 1.5 bar heating & hot water work fine but there is a constant drip from the 3 bar safety valve.
Initially I thought it was the valve and although the old one was knackered with a new one fitted the drip is still there albeit slightly slower now.
Any ideas please?
I see the manual suggests it could be the hot water exchanger pressurizing the heating system? does that makes sense?
The drip is there whether system is working or not.
Help please.

 

[franky]

Alright man so the only thing attached to this pipe thats dripping is this one pressure relief valve thats letting by.
The pressure gauge is right if you add a bit of pressure it moves slightly ?(probably wouldn't be this anyway as it would be more then a small drip). Have you let pressure out of the valve so it snaps back into place?

 

[Billy Bob Bob]

check your filling loop is not passing check what pressure you have in the expansion vessel it could well be faulty the dia does not have a plate heat exchanger it has a colirifer (the large grey lump) while this could have failed its unlikely

 

[PSABIN8951]

Ok guys, ive drained system, snapped valve back and refilled - no leaks from valve at this point. The leak only occurs when the system fires up so i'm thinking its definately the expansion vessel at fault. Does anyone know if the one fitted as standard on a DIA 20 can be recharged in situ? or would I be better off fitting an external vessel?
Thoughts please