Well guys, yesterday my new (to me) ZT-T started changing gear with an astonishing roughness - most notably when shifting from 2nd to 3rd.
I've only had the car 3 weeks, and previously my only other automatic was a 75 diesel now owned by Roverlink that never put a foot wrong. I'm a little out of my depth!
I'm going to replace the solenoid, and produce a guide detailing how it's done including plenty of pictures. I had trouble finding similar myself - so hopefully it'll help others doing the job.
Here's a pinout with the values you should get - to test, you need to locate the twist-together connector between the wing and battery box. It's the connector towards the front of the car.
I'll add more to this when I've done the swap - ordering a solenoid tomorrow.
Once done, I'll request it's moved to the 'How To' section for future reference.