As promised, here's detail on this.
I bought a "meat thermometer" the kind used in cooking with a sharp probe to measure the inside of some meat, chicken etc. to see it is being cooked right. Range is around 0*C to 300*C. The probe's lead is around two feet in length which needs to be extended to suit the vehicle. I used thin screened lead as the probe already has screened lead. You'll need to cover the original braiding adjacent to the probe with some plastic pipe to stop water getting in. The lead was fixed down with gaffertape/duct-tape onto the undertray. Keep it away from the hot exhaust! It's a conventional thermistor probe with negative temp. coefficient measuring around 100K at room temps. It is a swedish "stecktermometer" with a nice brushed aluminium face, various functions, plus a handy temperature alarm, and cost £10. It has a convenient little extending flap at back which can be pushed into a fascia joint, in S-type case in centre just below the heater vents. I arranged the lead to go outside the car, running along the joint between the front wing and sill (or cill). No holes drilled anywhere! No tapping into the ATF circuit! No tapping into the car's wiring! It runs off an AA battery (or cell).
Experiment showed the best mounting place wasn't on the g/b sump as I'd first supposed, but on the top fluid pipe on the g/b where it goes to the cooler. I fastened the probe to the pipe with tie-wraps (US zipties?) of the fairly hard plastic/nylon type that won't melt. Some Baco-foil around the joint + some loft insulation to keep the probe free from draughts, and away I went.
First impressions showed that the Jag. temperature design for the box was as said in that long posting in the diesel section, around 90*C for normal running. This is where there seems to be a departure from what the boffins say. They say the best temp. for ATF is 175*F or 79*C, this gives it the longest life. So I'm wondering now if there is a simple way of stopping the hot water flow to the cooler where exists a two-way thermostat? Anyone with insider knowlege of this and just whereabouts the two-way thermostat lives please tell!
Leedsman.
p.s. If anyone makes one of these for hot desert conditions, let us know what you think. It never gets that warm in Britain.

 

That should also be useful for refilling the transmission after an atf change.

 

See AmazingRibs.com for good electronic meat thermometers that could be used. When you're through, you could use'em to cook dinner.

You could also use the following very inexpensive thermometers to measure the transmission oil pan temperatures:

Non-Contact Pocket Thermometer

Infrared Thermometer

Non-Contact Infrared Laser Thermometer