Thanks for all the info and I've had the beast up on a ramp, where evidence of an oil leak from the transmission was plain to see.
The backstreet garage guy replaced two dodgy O-rings, cleaned the underside & replaced the (old & black!), fluid with new Dexron. And fixed a piece of loose trim while he was under the car - which now runs as it should.
Who said the age of good service was dead?
Thanks again - the data will be useful if & when I have the gearbox filter replaced next year,