If its sparking well, the fan must be running and will have proved to the PCB that the air pressure switch is working or the PCB wouldn't call for a spark anyway.
If its sparking but not igniting and simply moves to reset with a flame failure error, the PCB can be checked to ensure there's 230v at the board for the gas valve. If not, the PCB is likely to have gone.
But if there is voltage here, are we still then getting voltage at the plug end of the lead which connects to the Gas valve? If not, the lead has gone, but if you are, the gas valve is likely to have gone. My thoughts are that the gas valve has gone and needs replacing. Approximately £90 for a new one. Fitting should be around £40 for an hour's work.
Does your engineer sound like they know what they're doing? Dunno, check the Gas Safe Register online and confirm they're registered to work on CEN1 and that they're currently in-date. You can also check on the back of their Gas Safe pass for the same information. Remember, it's illegal for anyone who's not Gas Safe Registered to work on Gas appliances...
Chaumont Gas Services
February 2011