+1its muck in your pilot basic fault get it re-serviced pilot lifting off the thermocouple when main burner comes on
Burner pressure alone does not show up restriction in gas supply.
Needs inlet pressure checking at gas valve as boiler fires.
Tony
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local