How can i fix my Worcester Bosch 28Si II?

Hi my boiler started flashing once per second and i've just been reseting it on and off for about 2 weeks putting it on my list of things to get sorted. Today the light was flashing 5 times a second. When i reset it i get the burner kicking in as normal then about 20-30 seconds later get some clicking relay noises and the burner light keeps going off and on intermitently then the 5 times a second flashing light. I'm just after advice of what the problem is so i can price the part if need of replacing so i dont get stung on a price for repairing it.