Hiya Mazza
My first port of call would be a drop of soapy water on the fill valve end while the gun is under pressure, after that dont put any air into it until you are certain it has dried out, if you get bubbles you have found your leak, but do be sure to depressurise or leave it to stand til the pressure dissipates, dont et excited and try to strip it under pressure or it may get a bit lively.
Parts are easily available and its a nice easy job to do.
Best of luck