OK, here are the results of tonight's testing;
First I took the back covers off again to take photos (which I will attempt to upload later) and to check connections, loose wiring, etc. Everything looks solid, no obvious dry solder joints, no exposed wires touching the shielding paint. Under the trim pot cover I checked the 3 brown pick up plugs. Again, all seated firmly but I wiggled them home to make certain. I did the same with the black multipin plug with the rainbow ribbon wire. Again, all seated nicely.
So, onto testing the pick ups...
With battery power only, using a standard 1/4 lead and using Mica's screwdriver test with both filters and CVQs wide open, here's what I get with each selector position;
Position 1: bridge pick up only
Position 2: both pick ups
Position 3: Neck pick up
(interestingly, listening through headphones plugged into my amp I also now heard more tonal variations when using the filters and CVQs)
same test using the DS-5 set to mono with a standard 1/4 lead from the bass output into my amp, filters and CVQs wide open;
Position 1: no sound
Position 2: neck pick up
Position 3: neck pick up
And just for comparison, still in mono but with the lead from the DS-5's treble output into the amp;
Position 1: bridge pick up
Position 2: bridge pick up
Position 3: no sound
So in summary, using battery power I seem to be getting sound from all 3 pick up selector positions. However using the DS-5 I'm getting one or the other but not both at the same time.
I haven't yet been able to play through the amp at loud volume (sleeping kids and all that...) to test the sonic variations but it would seem that battery power might give me a better sound palette but the DS-5 isn't happy. somehow...
A couple of final points to note;
1) The mono/stereo switch on the front of the DS-5 got broken off in transit from Germany but I checked that it's set to mono by using a small precision screwdriver to make sure it was flicked all the way down.
2) I looked inside the DS-5 and nothing seems loose or out of place. Again, very solidly put together and soldered.
As I mentioned earlier, my pick up selector has 3 clickable detents, i.e. from it's furthest position counter-clockwise I can click it into 3 positions clockwise, so the 'zero'/'rest'
position is off. Interestingly the black knob seems to be upside down, meaning that it's pointing down towards the filter knobs rather than upwards, but I guess it's just been refitted this way - the switch still rotates the same way.
So there you have it. Not sure exactly what the problem might be but using the DS-5 is causing some issues, whereas battery power is giving both pick ups.
Or maybe I'm just being completely stupid somewhere along the line...! :-)
Thanks once again for your kind interest, help and guidance.