UPDATE
Thank you all for your help,
I was recommended to visit a garage locally which was formerly a small market town Toyota franchise. A good sign was the number of Toyota models in the large workshop.
After discussions with the owner (40 years Toyota experience) he has advised that the coolant loss is most likely a slow weep from the water pump (as diagnosed by Carolyn) Apparently it drops into the hot engine bay and evaporates, hence no trace anywhere.
It is booked in for a couple of weeks time so i will confirm the outcome then.

I agree about the later cars rusting more, but think the market will just focus on the late final editions.
These cars have always been underrated which is a shame

The problem with late facelifts is they are the worst for rot underneath.  I predict, when these cars really do become collectable, good PFL cars will bring the highest prices.

Unfortunately they aren't demanding that sort of money

A late facelift with factory hardtop and AC with very low mileage plus a hand full of TTE extras that's been kept in garage might get close

I don't believe any age Roadster is worth 8k unless it's along the lines of ex @Bernie or @MisterK cars.

TF112 is for sale on eBay for £7995. It has a rather poor MOT history with a mileage discrepancy in 2010.

I don't see how it's worth £8k.

The car not starting means the gearbox is not in neutral. I have a list of possible SMT error codes and P1646/P1647 is not among them.

I can only guess without seeing your car, but unless you have an SMT fluid leak that's leaked over the SMT sensors, you must have a faulty ECU.

Is it possible that your soft top drainage pockets have gotten clogged up at some time, and water has poured over the ECU?

I just think that, just because it's an SMT model, you're focusing on it being an SMT fault when it might not be.

Nice new one in place 😎

Just a quick update. I received a brand new Denso radiator with next day delivery, great service. Also, glad I made the decision to renew it. The photo says it all I think. It is 20 years old I suppose. The thing is it's an item easily overlooked as its not very visible until you remove the fans or peer through the front bumper. No sign of overheating on the dash with this but I reckon a leak was imminent. Maybe take a look at your rad if you haven't already 👍.

I don't have one, but did repair one after it came back 'dead' from a garage.
I don't like pushing people away from this site, but your best source for help is the Facebook Spyder SMT page.  https://www.facebook.com/groups/656921144477464/ There are a few on there that can answer anything.
To diagnose it you really need Techstream, - you'll get help on there as to how to get it.