Potterton Prima F60 firing problem?

My boiler is experiencing start up problems.

After the fan starts and electrode sparks the pilot light does not ignite for anything up to 25 sparks. Once the pilot does light the main burner kicks in and the boiler runs fine ie once running the pilot kicks in after 1 spark after call for heat.

I have looked through this site extensively and could not find a similar problem.

I suspected the pilot to be blocked after reading the boiler manual but I found by actually lighting the pilot it would fire.

Could it be a partially blocked pilot or even a gas valve.

Any help would be greatly appreciated.