Worcester 24i Ignition problem, new igniter?

Hi,

I run a shower direct off a worcester 24i, and it goes well, except every so often it doesn't heat the water until you turn on/off the hot tap several times.

Just doesn't seem to be igniting.

You can hear it making a "click Click click Click" noise when you turn the hot tap on, but it doesn't make a bassy "boom" noise that it makes when the gas ignites and warm water is produced.

Any ideas please?

-Andy