This is a bit like my current problem with a Halstead Finest (Wickes 90).
DHW, no problem. CH, lights fine when cold, doesn't always re-light when
warm. They do three re-tries and with the covers off you can hear the
ignition spark (about ten regular clicks in three seconds), and then lock
out. Irregular clicking sounds like thermal expansion to me. If you have a
DVM and don't mind playing around inside you might find it interesting to
hook up to the "modureg" valve which throttles the gas inlet. On the
Halstead, the voltage modulates from 2.5 to 20 V according to the book,
actually 4 to 18 on mine. But it should set to about 10 V for a few seconds
during the ignition sequence. Drops back to ~ 4 once it is alight, and ramps
up to full power over the next minute or so.

On mine I have just changed the board and it is better but still misbehaves.
I've also found my thermistor is out of spec and when the new one comes I
will measure it and post the data. If you *do* decide to change the board,
give Geoff a ring (www.cetltd.co.uk) and see if he can do you a
reconditioned one. About £55 instead of £200 in my case. Highly recommended.