Combi boiler fires then locks out then fires again?

My Sabre combi boiler has started to play up, to fire up you need to remove the sealing panel from the front, if panel is causing an air tight fit only the fan will come on, no gas or sparking (both work when panel off) but when panel is removed the boiler will fire up from cold stay on for 10 secs then switch off and fire up again, then stay on for 10 secs or so and keep doing the same. Sometimes you get lockout for 30secs. Now guessing this is a sensor problem, have cleaned both venturi pipes no different, not willing to do too much more to boiler as not gas safe trained. Any ideas appreciated.