Hot water issue with Worcester 240 Combi BF?

I'm having a problem with my combi boiler. i can only get warm water (not boiling) when i have the heating on.
when the heating is off i need to turn the tap so it's nearly fully open before the burner on the boiler clicks and kicks in but then the water flow is so fast it doesn't have time to heat up. if i try closing the tap a little, the burner just goes off.
Before, i only had to turn the tap on slightly and the burner would ignite straight away and i'd have hot water.

any ideas what it could be??