we drained system to fix a problem, have refilled, but boiler on stays on for a few minutes and then turns itself off

Must be airlock in system.... checking all radiators full open valves & bleed air out.

Sounds like there is air in the system and this is causing the boiler to shut down. Is there somewhere to bleed the air from the system, usually within the boiler area but certainly on every radiator. I would check these for air then try restarting the boiler when your happy all the air has been bled from the system.

sounds like air bleed all rads untill water appears should solve the problem