Baxi Boiler IgnitionProblems

[jasm70]

following the servicing of our Bax Bermuda Inset 2 boiler, the boiler made a gas explosion bang as it ignited in the eveing, this occured twice. The boiler also seems to light periodically rather than stay constant for the period it is on. It also ignites when the cycle has finished.

I have read other sites and the inginition could be the pilot light not able to ingnite the boiler and allows a build up of gas before it does or the wind can cause a vacuum that stops the pilot light igniting the gas??

The thermostat is set as 21 degress,all but 2 rads get hot (when I have bled them the 2 that do not warm up chuck out black water,when all the others are clear). the pump is on setting 2 and the water cylinder is on 160 degrees.

System needs flushing, but what about the boiler?