Ideal Elan 2 boiler cutting out and back on every 15 seconds?

I have a Ideal Elan 2 boiler that must be around 20 yrs old. Was in the house when we moved in. I recently had a problem on an unrelated issue and the boiler was turned off (gas turned off) then all turned back on again. Since then it comes on for 15 seconds, off for 15 seconds and so on. The pilot light stays on all the time and when I had a gas engineer, plumber and electrician all look at it they say it sounds like it's kettling, then switching itself off. None of them knew what was exactly wrong. Some say the boilers integral thermostat others say gas valve. But no answers yet. Does anyone have any ideas please. And where can I get parts for it should I need....thanks.