Recieved an e-mail from Elkparts (those guys were still working at 6:45pm on a Fri night).
There are a couple of tests for a working APC valve.
1) The resistance from the centre pin to the ones either side should be 2-3 Ohms.
I checked this and it was 2.5 on each side. Same for my old valve.
2) You should be able to blow from the C to W connector but not from W to R.
This didn’t work. C to W is sealed and W to R flows freely.
I believe this means the piston is jammed. I don’t understand the workings of this valve, but the effect doesn’t seem to be good. I will therefore put my old valve (which doesn’t hold much boost) back on until a replacement arrives.