Hot water taking long time to heat from boiler?

Hi, I have a Potterton Combi boiler, recently the hot water takes alot longer than before to arrive once I turn the tap, the time varies, sometimes upto 45 seconds.

I've opened it up and found that the gas isn't burning straight away as it did before.

i have also found that the time really depends on the flow rate of the tap, so if i keep on adjusting the flow rate, the burner light comes on eventually but can still take a while.

I've had an engineer come out, but unfotunately I wasn't in at the time and he said it was working fine just got to be a bit patient! I don't want to be wasting all that 45 second's worth of cold water just to get to the hot water!

Any suggestions?