Worcester 24i intermitent?

My 24i wont come on in the mornings, the pilot light seams to light very weak then goes down/off. After flicking the electric on and off a few times its ok and stays on fine, that is until its set to auto and may or maynot refire.

I had a plumber take a look and hes not 100% sure, I guess hes not familiar with this boiler.

I had it serviced and the fan greased up (aparently it was a little dry) and every morning the heating is off. And we all know how cold it is at the min....

Any advice is greatly appreciated.