Well I changed the o-ring and after reassembly it still leaks. On closer examination it turns out the weld around the tap on the tank has failed, a tiny 2 mm long crack. I have chemical metalled all around the weld to completely cover it, waiting for it to go off now to see if it still leaks. Have had great success in the past with chemical metal one to repair a steel tank that was leaking from rust and once to repair an engine casing that had a hole from a rock.
ridelimousin's post in Fuel Leak From Top Of Tap Txt 280 was marked as the answer