where is the pressure valve on worcester 28Si?

spotted my sons boiler pressure gauge on almost 0!! I have a new condenser worcester and know where to fill mine up from but what i think is his has had an epoxy?sealant put on it to stop it leaking and so will not turn. If i'm right it is a small black cap at the end of a pipe protruding from underneath the boiler near to the pressure gauge.Any suggestions!!