Struggled to get over 80degrees, turned on ECO (turned off compressor), temperature got up to 90deg or so.
Where do I look then!
My guess would be a thermostat that's not closing fully. Switching the air on on will start up the auxiliary cooling fans resulting in more airflow through the radiator and a cooler engine, if the engine temperature is not being controlled properly by the thermostat.
Try starting the engine from cold and feeling the temperature of the top radiator hose, and also the engine temperature at the heater hoses (with heater on). The heater hoses should warm up gradually, and start getting warm pretty much as soon as the engine starts.
The top radiator hose shoiuld remain cool (it may warm up a little) until the engine has reached normal operating temperature and suddenly get very hot when the thermostat opens. If it warms up with the heater hoses I would suspect a dodgy thermostat.
If the air con affects the temperature gauge immediately rather than after a few minutes running it could be a bad earth to the engine block, with the extra load from the fans affecting the reading from the temperature sensor in the coolant bridge.
Kevin