Ok an update. Was lucky enough to have found out that a mate has a mate which is a fridgy and a good enough lad to came and had a look at the A/C unit. And spent about an hour looking for a leak and found nothing as well in the condenser, piping, compressor and evaporator? But what he did find was that the Schroeder valve in the test point was leaking. He replaced it and pressurised it to 300psi again and will be leaving it till next week and come and check it to see if it looses pressure? Fingers crossed that's all it is!