Thanks for the advice about checking with a straight edge as a matter of course, and using a hammer and wooden block. Would have saved me a lot of hassle.
I received the new transmission pan from Singapore within 48 hours of the dealer putting the order through. Pulled the old one. It was instructive seeing old and new side by side – the new was perfectly shaped to the transmission, and the gasket sat perfectly. The old pan was really out of shape and several of the curved bolting areas were almost flattened. Not surprising I couldn’t get it to re-seal, even with a new gasket. Someone had already done the hammer trick on it. At least once! And with a bit of corrosion as well, new pan was warranted.
Offered new pan with gasket up, and progressively tightened the 6 bolts from hand tight through 2, 4, 6 and finishing at 8nm. It has been on for nearly 2 weeks now, and not a drop of ATF to be seen where it shouldn't be.

Before servicing the car had a problem, which has continued. It shakes when drive is engaged and when stopped at lights etc. The shake feels like a clutch slipping in a manual, but it's an auto. Can feel a bit of vibration in steering wheel and driver’s seat when driving at 50--60mph (3000 rpm). Still have the problem, and it varies in intensity from barely noticeable to 'change, light, go green now, get me moving again!'
Car starts easily. Engine idles smoothly and consistently in park and neutral. Reverse engages smoothly. and quickly, and drive engages quickly. Shifts smoothly when rolling, engine runs smoothly.
I’ve done some searches here and they led me to the engine mounts. Have inspected them and the rubber is cracked. Really cracked. Will install replacements over the weekend. One mount is accessible, the other (passenger side in my right hand drive) looks impossible to get at, but everyone says to persist. I won't start a new thread with this issue as there's plenty written already. I hope it's the mounts, not the transmission ...
John