Thanks to a very helpful guy on Strat Talk, I now know what was wrong. I used a no-load pot for the blender - & it was the no-load pot that caused the problems.
I switched the no-load pot for a standard Alpha 250k B (linear) pot, wired it up exactly as before, & it now seems to work fine. The circuit now works as described in the Acme diagram - series wiring in positions 1 & 5 when the third knob is turned down to 1, & standard Strat wiring when it's turned to 10.
The series sound is fatter (as expected), & time will tell how much use the series options are in practice. But of all the extra wiring options that are possible on a Strat, this one seems like it will be the most useful as it gives the option to fatten up the sound when needed. And all for a maximum of ?2.20 - the cost of a new Alpha pot. I imagine that it would work (though maybe not so well) with a log (A) pot, but I haven't tried that yet.
So, if you try this yourself, remember that you can use a no-load pot for a parallel blender, but NOT for a series blender.
If I can get myself organised in time, I'll try to make sure that I bring an example of each to the Leeds convention in a few weeks - one Goldie with a series blender & one Goldie with a parallel blender. It will interesting to hear what other people think of the two options.
8)