Intermitant fault with worcester 20/25 ?

hi can any one help, we have a worcester 20/25
since the enginneer visted 2 months ago to do the service, it hasnt been fireing on demand, its been choosing almost randomly when to give hot water normally enough hot water for a shower but never more.

Can any one tell me whats causing the issue, and whether there is an easy fix at least then i can try to get a different engineer out.

The filter has been cleaned on the oil tank, and there is 900ltrs in the tank and the tank has not run dry prior. any help or advice would be greatly apprieciated..