Mike and Christopher, good to hear I'm not alone. It's a pretty cramped job, even on 4 x jack stands. I used an OEM filter and gasket so no materials issues. I've just been out and drained the tranny, removed the pan and checked everything. I think I've found the problem -- the bolt by the leak is partly cross-threaded, and it has distorted the pan a little which in turn has affected the gasket mating surface and caused the leak. I didn't think I was using excessive force -- far from it -- but must have done so. And looking at the picture of the new AT pan on Fastlane I can see that many of the bolting areas on my old pan were bent out of shape by previous owners before I had a go at this job. Happy 20th birthday, car.

So,a new pan coming right up. I think I'll also need to tap a new thread and fit a larger bolt, which I'm a bit nervous about doing around the transmission (cleanliness v. metal shavings). I'll keep you posted. Fixing and servicing this car is like playing dominoes. Still, better dominoes than roulette.
John