HeatLine Compact Boiler locking out?

My HeatLine boiler locks out on a daily basis after working perfectly for a while. When reset by switching off power and starting from scratch all seems fine, but on the next timed cycle will cut out again eventually. Heating and water are set about half way, not too high. Latest engineer (from B Gas) unable to say conclusively what the problem is - suggests a £570.00 powerflush may solve it! Any help, please?