Micron 60ff boiler. Ignition failure?

On most days, usually in the morning and evening when the boiler is programmed to come on from cold, this happens:

The timer is programmed to turn on the CH and HW from 06:00- 09:00 and 16:30-22:30.

The fan starts. APS operates, the ignition sequence starts (I can hear a tick-tick-tick noise), but the burner light does not come on, the boiler then the switches itself off and starts this process again until it decides to give up after 3 or 4 attempts and shows a solid green light and a flashing green light which, according to the manual, means that there is a ignition lockout. The interesting thing is that there is an extremely loud bang just before the ignition sequence starts. (There is a loud bang if the boiler starts or doesn’t start) The BG engineer says that it is nothing to worry about. The bang occurs just before the spark is generated and sounds mechanical; it is not the build up of gas igniting! I explained to the BG engineer that it has been making this noise ever since the ignition problem started, but he doesn’t seem to think that the noise is related.
It seems to work fine when it has been running for a while, it’s just the initial start up that proves difficult and requires resetting a number of times before it starts to work. I reset the boiler, either by turning the thermostat to min and then to the normal setting again, or just by switching the main off.
The boiler has had a new PCB about 5 years ago. The British Gas engineer has replaced the electrode and HT lead. The engineer was called again this week and replaced another HT lead, but that also hasn’t solved the problem.
I will be calling him out again, but just wondered if anyone could shed any light on this. There seems to be a lot of people who are experiences something similar with this brand of boiler, but I haven’t been able to find an answer to this problem.