brick wall...
I've narrowed it down to (theoretically) two garages, I've called the Dealers who bought those sites, Lookers, who have nothing on their system, put through to Vaux's Customer Services, who couldn't find my VIN on the system (!) and so werent able to help.
Had a brainwave - the car was, as part of a Fleet, of course serviced by a Main Dealer until 2002, this would in theory be the same Dealer that sold it in the first place. If they have even one record of my car being serviced there, then bingo, that should be the original retailer. ... So called the physical site of the one remaining old dealer - which are no longer sales, but just a parts dept, the guy was very helpful, tried the Lookers and the GM system. Can prove the car
exists (thank God my car hasn't been fictitious for all these years)but nothing regarding anything else. Again, he can only search as far back as when they were bought out, so 2007ish, nothing before.
No dusty old ledgers with lists of cars serviced there, I did ask! Unless I can find the old service manual (neither me nor the previous owner are sure we ever had it) I'm stumped for now...