Vaillant VCW 242 slow to ignite?

Hi all,
I have the above model & have a slight problem, boiler is slow to ignite when heating is called for, I have timed it from when I turn on the thermostat on the wall, the boiler picks up the signal from the thermostat fine.

The pump kicks in & you can hear the fan kick in & working, then about 20 seconds later you hear a sensor or something make a noise & the boiler fires up, this takes about half a min, it used to take about 5 seconds.

When I turn of the hot water taps the boiler fires up in about 3 seconds.

Anyone got any ideas?

Any help would be appreciated.