Actually, Adrian's gauge turned out to be an odd failure - the SPG was working properly, the face and plastic housing had just seemed to come apart, and the face had rotated about 90 degrees clockwise. As a result it looked like it was displaying 1100 PSI when really there was around 2700 in his cylinder. There was a slight dribble of gas from the spool though, so we ended up just replacing the entire gauge.