If the boiler will not stay on after ignition, it can be a few things. Will be best to get an engineer to take a look. To save abit money try to get an engineer that does wotk on the side outside of his normal job.
It should only be Â£30 - Â£45 to diagnose fault, if its a simple repair, like setting pressures etc he'll probably do it with no extra charge.
Any other faults and he'll charge for parts and fitting.
Does the main burner flame come one ? Sometimes the gas pressures need resetting, This happens if the low burner setting is to low, and the sensing electrode is not detecting it. But
flame sensing electrode fault, not sensing the flame
gas valve fault, gas valve not opening for main flame.
PCB fault not opening gas valve for main flame,
Fan or pressure switch fault, fan not going to high speed.
So its quite hard to diagnose without seeing the boiler.