Well I went to Halfords and bought a 20 quid can of combined refrigerant, oil and leak sealant. Put the car onto max height and set the aircon running. There are two valves, one on each of the pipes going to the compressor, however the nozzle on the can only fits one of them so that solves that question. Give the can a good shake, attach to valve and press button. After a while prize frozen fingers off can.
And ......... hey presto cold air coming out of air vents. Also I only seemed to use half a can so I've got some for later. I can see coolant circulating in the sight glass on the thingummy whatsit under the bonnet and I've been for a half hour drive and it's still cold.
Result I'd say