It sounds like a simple ignition fault
It could be caused by semi dirty electrodes not creating a hot enopugh spark to light the gas every time
Are you able to take the cover off and clean them - do you have the manual to show you how to do this?
Andrew, sorry I know it's an ignition fault. Very unlikely I think to be the electrodes as the system has been serviced recently. Don't these boliers do an airflow check before firing up? Might it not be a fault with this area.
If it is not getting to the ignition stage then it could be the Air Pressure Switch
Andrew, might the fault lie
If the APS is not triggering then you should try cleaning the fan and connecting tubes to the APS and see if it helps to start with
P.S. Not all "services" include cleaning the boiler......
Thanks for that, I'll see what can be done with the APS,
Once you have cleaned the tubing then put it back together and see if the APS clicks when the fan runs - if not then take the tube off the fan and blow/suck very lighlty on the tube and see if you can make it click yourself