engine overheating mainly when running A/C, otherwise ok

Several things come to mind. Firstly, could be your aircon pump is starting to seize up, putting drag on the engine. Secondly, if the engine and rad are clugged up, putting on the aircon could be the last straw for the cooling system. I'd also check that the thermostat is not partially stuck, it may be open enough for normal working but the strain of the aircon is putting it over.

PDH

It sounds as though the radiator is partially blocked. You probably need to flush the radiator, or possibly replace it. This is common where the factory servicing recommendation has been ignored (usually flush the system and renew coolant every two years)

mrcheerful

Check the function of any electric fans fitted to the radiators.

Has it got an auxilliary electric fan and is it working? Is the condenser blocked with leaves, etc? Is there a locking viscous engine driven fan and does it lock up?

Dunno about your car specifically, but on cars fitted with air-con, there are usually 2 radiator fans. When you switch the air-con on, usually one of the fans comes on and stays on until the air-con is switched off. Check and make sure the second fan is working (If there is one) by visually checking when you switch the AC on...