ariston sx20 mffice not working?

when turning on heating or hot water i can hear fan running but no flame. if i remove main front cover with little glass window in it (the one held on with 7 screws)as soon as i have most of the screws out of this cover and there is an air gap the boiler will ignite and start to work ok .but as soon as i re-fit cover boiler will cut out and only the fan will run.i have removed the flu pipe to outsie the house and it is clear/not blocked,and removed the fan and cleaned it/checked for blockage.also both pipes to duo sensor ok.also can hear micro switches operating if i gently suck/blow pipes to sensor.
thanks for any help

mark s
October 2008