How do i fix my Worcester 24 si 11 combi?

The main difference between hot water and c/heating is size of the flame as the c/h can be set lower than the maximum for smaller properties. It modulates for hot water as well but only when the water is quite hot.
It may be that the low flame setting on boiler isn't quite right and the flame rectification system isn't picking up the signal.
Your engineer should be able to test for this.

T.H.S
May 2007