Greetings everyone, time for an update. Thanks for all the helpful comments so far. With respect to Grumpy's latest suggestion, it turns out that I had in fact blown the green 20A fuse in the engine compartment fusebox, explaining why I started getting nothing at all when I turned the key, but replacing it did not fix the original problem.
Just before Christmas I was put in touch with a Discovery enthusiast who works locally as a mechanic for John Deere, who agreed to have a look last week. He swapped out all the relays under the hood and tested the battery and alternator; took out the starter, exposed and inspected the brushes and liberally shot a WD40-like cleaner throughout the assembly before re-installing; replaced the clip of the exciter wire with a new one (I may be translating that incorrectly, it's the wire going to the small tab on the outside of the starter); and carried out a number of other checks and tests. In all he spent five hours on it without finding anything concrete. The engine would start more frequently with the key, but still nowhere near reliably. In the end he put in a double tab contact at the starter for the exciter wire and ran a wire from the exciter contact through the firewall and to a momentary button (fuse protected) inside the glovebox under the steering wheel, with a second wire in from the battery. If ever the car won't start with the key I have this button as a back-up.
When I left to go home I had to use the button to start the car, but since then it has started with the key every time.
I wonder if it isn't just the battery after all. The reason it wasn't starting reliably in my friend's garage could be just that the battery was a little drained from all the attempts at starting even though it was still turning the motor over; could be now that I'm driving it and keeping the battery charged it's resisting the load better.
I tried to test the battery's amperage with my multi-meter this morning by attaching the meter in series between the battery and the positive cable (disconnecting the cable from the positive post and attaching my meter so as to make the juice from the battery flow through the meter before reaching the cable) but I got no result when I turned the key so I must have been doing something wrong. I put the cable back on the battery and was able to start the car no problem.
We've been in touch with the president of the local Land-Rover club (whose existence I knew nothing about until now
) who will be in touch if he can think of anything, and maybe one of you bright sparks will think of something to chase down but for now I'm happy to leave it be until and unless the situation changes somehow.
Thanks again for all your input.
