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I must admit i am not sure on the Greenstar. but Hard Lock outs for Riello Burners are usually cured by removing the power for 10 seconds. you must Isolate it completely. Turning it off by the timer will not work.
Excuse the poor grammar and spelling please my system is resetting every few seconds and i cannot correct
There should definitely be a mains switch in the house but if you can take out the power at the boiler by unplugging it or similar that should do the trick or maybe there is a boiler fuse at the consumer unit