I'm posting this in case it helps anyone else with the same problem.

Some of you may have followed my earlier thread re my carby engined TP auto being reluctant to change up unless I backed right off the throttle. I checked everything on the transmission and concluded there was nothing wrong with it. All the symptoms pointed to the TPS not being correctly adjusted but I had checked that again and again.

Well I thought about it for a while and realised it could be related to another common Magna problem that I had - the secondary throttle on the carby not working properly. I will post separately on that matter but yes, that was it!

Because the secondaries were not working, the amount of throttle required for a given amount of power was much more than it should have been. Even though I was driving very conservatively, I was unwittingly using quite high throttle openings. The TCU (through the TPS) interpreted this as a high power situation, so it held the lower gears, especially 1st gear, far longer than warranted by my actual driving style.

Now that the secondary throttle is working, far less throttle is required and the transmission changes up perfectly. So I've killed two birds with the one stone!