Can anyone help me with my Myson Boiler?

I have a Myson Back Boiler and it is 21 years old, it ignites when I put it on and then gos out, after a while it starts clicking to ignite again but it maybe does it sometimes up to 9 times.

The hot water is hot and the heating works really well, we had a local heating engineer out and he put on a new printed circuit board and when he left it was igniting after a couple of gos, does anyone know why it does it.