The header connector CP07 can be found in the bottom right corner of the main PCB. Ensure that the relay and diode are mounted in such a way that none of their pins can come in contact with any metal within the CPC housing or any other part of the main PCB.
The transistor and diode shown in red below, must be added to the main PCB, the pad positions should be present but unpopulated.
The pinout of the 2N3904 is identical to the original transistor used by Amstrad and should be mounted as shown below.