If you're thinking about PXing, I would agree a deal before, subject to inspection. I was given a PX offer which migh as well have had "you're stupid" attached to it, there and have subsequantly read that my experience wasnt unique. To give you an idea, I asked a mate in the trade to give be bottom book (i.e. the worst condition for its age/milage) and it was MORE than I'd been offered for what I considered to be a **** near spotless car.
In terms of ADG themselves, you're absolutely right, if you want a GS, you cannot ignore them - they seem to have the market pretty much sewn up when it comes to GSs. Their prices seem to be high to start with, but quickly become more realistic.
I'm not 100% sure and perhaps I'm putting 2 + 2 together and getting 22, but I seem to think that they had a new partner when I was there who has subsequantly moved on....they always have good stock so do something right somewhere along the lines!
As for inspection, personally, I would want the car inspected 100% independandtly. Generally, I believe you take the car to the place for inspection so they'd have to be willing to do this. If they're confident in the car, they'd be happy to olbige. Its worth remembering though that you're buying a used car so remember that the inpection lets you know what you're buying, it is not to provide a fix list for the vendor to address and to get the car back to A1 (IMO).
Best of luck with the hunt. Of ADGs current stock, I like the sound of the one owner Rosso Mondial. It that's been serviced on time and looks to be well loved, thats a hard one to walk on by