Since last writing I've had my system checked out by Aircon Direct. Fabulous service from a very experienced engineer. Fortunately all is well and I'm still on R12.
The only issue is that there is little cold air coming through the centre vents. Plenty through the lower vents and the screen vents. I understand that it could be a solenoid problem?? Has anyone any advise on sorting this out and is it one of those jobs only a pigmy with tiny fingers can do 
Blessings,
Chris
