worcester 24i combi20-30 minute delay between demand and ignition?

I have a worcester 24i providing central heating and indirect heat to the hot water tank in a sealed system controlled by 2 zone valves which has been running without probems for 10 years so this is not a system fault.
If i over ride the programm timer to turn the boiler on there is a 20 to 30 minute delay why?
tank & room stats are on demand