To get DVLA information register as a car park owner. You'll get details no bother.

Gauges are unreliable in accuracy, but good for a trend. If the lower hose is cold then it's probably okay, assuming the thermostat opens. The voltage regulator in the dash needs an earth and is essential for gauge operation.

Donno. Got a parts manual? Google the part number and you'll get the thread and dimensions.

Any photos?

Run the engine for thirty seconds without the belts. See if the noise reoccurrs. If not then it's probably the water pump, could be the impeller coming loose.

I can recommend the Haynes manuals as a start point if you prefer to read. The actual factory manuals can also be found in line and from booksellers. Britannia restorations, Mike, from Canada, on YouTube, is excellent.

No series engine has a timing belt. Sounds like it could be a fan pulley coming loose and catching the cowl, or an alternator bearing. Check the tension on the fan belt.

Main changes were better synchro hub teeth designs, but stronger? Not sure. Big changes though on some of the cogs, tooth count changed.

Plugs out, out of gear, turn over by hand. Starting handle or socket on the crank bolt.

What's the voltage regulator output like? 10v steady?

Petrol? Standard starter? Solenoid on the battery tray. White (& red?) wire from ign switch to solenoid. Battery - fat red to solenoid and another fat red to starter.

Some of their kit is excellent. The CVs that the coil sprung lot use are the best in the market. There's a few other examples out there. Most of their stuff is meh. And there's far too much crap as well. Infuriating, to be honest as a tiny bit more effort from them and they'd be a great supplier.

Britpart. Covering the full spectrum of quality parts. From rubbish to excellent.

I think Marshland were the JLR OEM. So there is that. Both are good enough. Which is cheaper and available soonest?

Wouldn't have thought so. There's a lot of movement in geometry with the spring length and shackle angle. Which bracket?