Thanks for replying.
It's not a manufacturer installed prv, I fitted the boiler 10 + years ago (replacement) and just left it on the pipework. It's a Grant 160/200 boilerhouse model , and now it's leaking I am wondering whether to remove it or replace? It's a 1" brass which costs £160 plus vat! If it's needed because of kW size then no prob but don't want to waste my customers cash.