This leads me to believe that the compressor is not starting , only problem
> is I have no idea if there is a solenoid or extra fuse for this
> Does anyone know what I should be looking for (as I say it doesn't sound
> like a gassing problem as the engine speed doesn't alter)
The compressor will be driven off an accessory belt direct from the engine (worth checking first!) via a electro-magnetic clutch. Presumably you could disconnect the clutch and apply power directly to make it engage (at your own risk!) to check that. Otherwise it could be one of the pressure switches in the system that prevent the compressor running under high or low pressure situations.