Starting issues on 82 S3 with 200 Tdi

I have an 82 Series 3 RHD with a 200 Tdi and am having starting issues. The rover has always started in the past, at times it takes 2 or 3 tries with hearing the started turn but it would always start. Recently the rover will not start. I'll put the key into the ignition and turn it and I hear a thunk/click sound. Everything goes dead until I take the negative lead off the battery and then I can do the trick again. With someone else turning the key, I placed my had on hte starter and felt it make the thunk/click sound. I assumed it was frozen or damaged and pulled the starter and tested it. For better or worse it works with leads attached and a connection is made with the solenoid on the starter to positive lead with a screwdriver. The gear pops up and the motor runs.
So now I assume the wiring is in need or repair or something else (starte solenoid switch as identified as #23 on the Series 3 circuit diagram as provided by our hosts online).
I'm posting on this forum as all of the Series 3 wiring system is there and unfortunately I'm not as familiar with it as RRC.
Any help is appreciated.
Thanks,
Mark

It should start within a half a revolution, so something is certainly out of order. Obvious things first; fuel starvation, clogged filter, injectors dodgy. Stop solenoid working properly. Timing spot on.
The starter may be bad if it can't spin the engine robustly. Check the grounding connection between engine and battery first then try a new starter.

My Series IIA did this - it was the starter shorting - or doing whatever it was doing so that nothing else on the truck would turn on or work until I unhooked the battery and then hooked it back up. Replacing the starter fixed it.

Sounds electrical to me. Solenoid sounds like it's doing it's job, I'd check the connections on the main wire and the ground strap as well as the battery itself. If that doesn't work, maybe have it rebuilt.