My pleasure, folks. I'm only mildly disappointed it took me nearly 30 years to work it out.
Anyway, having stumbled across it on my strat, I'm pretty confident that his 1989 sound was largely out of phase guitars - strats and the Gretsch. I dug out my spare Supertron pickup and put it in my Country Gent out of phase with the back Filtertron and had a play around.
It's tha alright. I suspect he might have had the tone switch in the up position some of the time, which is normally a very bass heavy sound, with his out of phase combination as you lose a lot of bass with the phase cancelling and that is exactly the sound you hear on, for instance, Sally Cinnamon live at the Hacienda.
So, yeah, a Gretsch with a bassy front pickup, wired out of phase, with the mud switch up is the sound of the Squire Gretsch and I'm 100% confident he had an out of phase option on his strat as well, but I think any two pickup guitar out of phase will get you closer than you've been before.