Heatline C24 virtually no hot water to Bath taps unless tap open only a bit?

Hi
The heatline 24 combi boiler supplies hot water to bath taps only when run very low, ie tap on only a little so it takes ages to fill a bath but is cold by the time it's filled. When tap is on half or full you don't get any hot water. Water to sink taps is fine. If you turn the central heating on you have a better change of getting more hot water out of the bath taps. Pressure is 1.5. Any idea's on what it could be?
Regards