how to remove key after refilling worcester 245Bi boiler?

I have used the bayonet key to refill the boiler, but can't remove it without water pouring out: I think the key must be damaged, and not locking properly. It is still in place, and I dare not remove it. It is allowing the boiler to refill very slowly, and pushing over the 1.5 limit. So far I have been draining a radiator to regulate the pressure, and looking for local retailer to buy new key. Can't go on like this for long.