Thanks SJW, I have already booked the car in to have the gas checked/re-charged.
I have checked for voltage at the clutch and found that there isn't any.
I had come to the conclusion that there may be a pressure switch in the system, but have been unable to locate one - presumably it is somewhere near to the evaporator and therefore out of sight.
Good, yes there will be a pressure switch in the system, if there isn't enough pressure the system won't work, most likely there isn't enough gas in the system. It will have either escaped due to age, or u may poss have a leak