When I was there (91-93) in theory both upper and lower showrooms were for new cars, but in practice they used to go through phases of having 'interesting' cars in the upper one. The rationale was that it was behind the Network Q stuff out front, and the intended front of house was actually the lower showroom. The 'interesting' stuff that I can recall, were all used, and included a then newish Nissan 300ZX, a Supra Turbo and Jack Halley's old Merc 420SE. The car park was where the non Network Q stuff went so the traders (and anyone else who asked) could see it.
The garage layout then, was upper and lower showrooms, workshop to the left (if you were looking at it from Homebase) with the parts department behind that; the yard, and the remaining old shed against the wall of the sawmill yard, was valeting. The petrol station shop was built onto the retaining wall when all that was excavated (in the 50s I believe), such that it was perched on top of a building you couldn't see from the road, which was offices at one point, but was storage when I was there.
We still had a septic tank there then, believe it or not! My how we laughed when one of the lads from Kelvin Timber dreeped over the wall to get their ball back, and went straight through the decrepit concrete lid. In summer, when the pit was long overdue a clean out.
The DIY shop started out as WH Smith Do-It-All, was Texas Homecare when I was there: the land belonged to the garage, because that's where the bodyshop and the parts dept used to be. The field over the back of West Of Scotland RFC where the caravans are/were was theirs too, as I think (and I could be wrong here) was part of Murray Park. They've made a bob or ten out of land over the years, safe to say. Probably still are.
Incidentally, if you look up stuff on George Bennie's Railplane, it ran from the sawmill, and some of the aerial shots give you a proper idea of how big that plot of land was: the sawmill and the burn were essentially the boundaries of it.
I'd agree that VX shot themselves in the foot losing a dealer like that: the short term gain of increased numbers (I was at Dickson's Honda in Aberdeen, when they lost that franchise to Anderson Cars in similar circumstances, so I get how that happens) doesn't compensate in the long run for customer loyalty, which they had in buckets. You just can't buy that, you can only earn it.
I'll need to go and have a look next time I'm over by: my mental map is probably well out by now!
The current layout is lower showroom is the Ford showroom now, if you are standing in front of the Ford showroom, to the left of that is workshops I think, on the upper level the showroom there is all pre reg Vauxhalls, because as I said Arnie are trying to pass it off as a proper Vauxhall and Ford dealer but in reality it's a proper Ford dealer and a Vauxhall approved service and parts dealer, pretty much all the used stock for sale is on the upper level, in some of the Homebase's car park it's used like a compound for new cars, used cars awaiting prep and customer parking.
This will give you some idea: