Thanks John for advice
But I think I have managed to solve problem (at least temporarily), bought some K Seal Permanent coolant leak repair from local car shop for £9.99.
Put K Seal in radiator overflow, ran car on drive for 20 minutes for it to work, no need for draining coolant/anti-freeze etc, took car for long test run.
Hey presto, problem solved, no white smoke. From internet research this solution might only be temp fix, someone said might last for 6 - 12 months. But for 19 year old car, worth most probably less than it would cost to have gasket professionally replaced, it will do me.
Taking car back to ATS to exhaust sorted out.