how do I ignite my boiler?

I am not qualified to answer this question - I can only guess based on what you say...

It shocks me that so many parts have been changed at your expense - whoever is doing it is just guessing and hoping rather than properly diagnosing.

The last thing that happens in this cycle to light the pilot flame is the opening of the pilot gas valve - the gas valve is a combined dual-unit - one valve for the main burner an an adjacent valve for the pilot flame.

The control board generates a spark and at the same time sends a voltage to open the valve - perhaps there's no spark - you can see it through the observation window - of perhaps the gas valve is defective (£££££)

After the pilot flame is lighted a sensor detects the heat of its flame and the main gas valve is then opened.

The allegedly defective parts belong to you
You might consider an alternative repairer

Continuing good luck...
November 2014