Worcester Heatslave 20/25 problem?

Worcester heatslave 20/25 oil combi boiler 8 years old problem as follows,

with hot water only selected on front panel when hot tap turned on hot water comes out ok but the heating pump also starts, radiators get hot.

With hot water and heating selected boiler doesn't fire up and central heating pump does not work unless hot tap is switched on, on this setting hot water comes out hot so is being heated.

Boiler has been serviced 2 months ago when expansion vessel was also replaced.

Any help on this would be appreciated