Last one I looked at with a similar fault found one of the coils burnt out in the BS box.

Link here for manual, page 117 for circuit diagram:
http://www.google.co.uk/url?sa=t&rct...8zROCIgdexO6Uw

Check operation of solenoid valves. Lower one is for subcooling and smaller so you can't miss it, no need to bother with this one. Of the other two, one energises in cooling, the other in heating.
Check you have a 240V output off the PCB (spade connectors) to whatever coil, probably have if you've changed PCB. Coil can be swapped over or stick magnet on to manually force open.
Strainer might be blocked if valve clicks open/closed.

Access is through the top panel so if it is tight to the ceiling good luck!

Cheers,
Andy.