I'm assuming that the HW really does work as expected with CH turned off and CH works when HW not drawn. When HW is drawn with CH on, you say the water is cold but does hot water continue to flow around the central heating circuit?
Check all the connectors are firmly inserted into the PCB and likewise on the diverter. Also check that the diverter head is firmly attached.
If the above is OK, check visually that the diverter head responds to a change in demand. If it does the problem I would begin to suspect the PCB which is failing to provide HW priority when CH active.
Is the PCB orange or black, and if orange how are the jumpers set?
Check all the connectors are firmly inserted into the PCB and likewise on the diverter. Also check that the diverter head is firmly attached.
If the above is OK, check visually that the diverter head responds to a change in demand. If it does the problem I would begin to suspect the PCB which is failing to provide HW priority when CH active.
Is the PCB orange or black, and if orange how are the jumpers set?