Definitely try Big Al (bus every 15 minutes from Buchanan bus station to his front door at Clachan of Campsie).
I bought an Airborne Valkyrie from Big Al several years ago (before they changed the stickers to read VN) and went I went to pick it up there were two others sitting outside, belonging to old-timers on a day ride from Glasgow. In other words, he sells a lot of these bikes.
And if I recall correctly Big Al's was listed by C+ as one of the top ten coffee shops in britain, which is quite an achievement for a bike shop that doesn't sell coffee!
(it's always on the go though - and his customer service is unique, but in a good way)
I'm in Stirling if you want to have a shot on my bike (I also own a Airborne Lancaster mtb) - they are lovely and you won't regret buying one.