potterton 80 loads of hw but rads luke warm at times?

we are at our wits end with this one.......

Potterton 80 Combi

the boiler runs fine and we have hw, trouble is when we get in at night and first thing in the morning the rads are luke warm, the rads have no air in and the boiler turns on and off ok but not for long periods just short blasts

its seems funny because the rads seem to get hot about 8 at night then keep hot till say 6 am ???? we thought it was the pump as the pipes to the rads were boiling hot but the rads again luke warm, we changed the pump but this hasnt made much difference

we dont want to get ripped off by so any body got any ideas please ?????? my husbands thinks it could be the diverter ???