My Worcester greenstar 24 ri is clicking?

Our boiler clicks almost constantly after the timed programme has ended. If we turn it back on the clicking stops but as soon as we shut it off again the very annoying clicking starts again! Only today the blue fault light started flashing but was off for longer periods than it was on. The manual indicates this is an "ignition lockout" but offers no tips on how to resolve this other than re-setting the boiler which we have done. This cleared the fault in as much as the blue light has returned to a static blue light without the flashing but alas, the clicking noise is still clicking away! Does anyone have any solutions before I'm forced to call out an engineer?
Thanks