how can I get central heating working?

The relationship between HW and CH on this boiler is very critical, you mention having the DHE renewed, this is probably NOT relevant. Assuming all external controls are calling for heat (240v on terminal 4):
When calling for CH does the diverter valve (bottom right of the boiler) make BOTH microswitches? If not, it is probably a problem with the water section/servo valve, the water section can leak through the stuffing box, stopping the servo valve from returning to the CH (rest) position.
1st point to start (assuming 240v on terminal 4)

Carefree
September 2010