In my experience a lot of barbel take a lot of moving once they find somewhere suitable to live. I used to see the same 6 barbel most mornings on my walk. These 6 were spread between 3 swims and all lived with chub (not a lot of barbel in the river). They hardly moved and as if to prove this scenario Jack Perk's the underwater camera person contacted me to see if I could put him somewhere to film a live barbel. I took him along and the water was not quite as clear as it usually was and the solitary fish that was always on its own and had a white nose on it couldn't be seen. I offered to guide the camera into place where it was always laid up by means of a tiny camera attached to a roach pole. it was filming blind but moments after Jack getting home and downloading what his cameras had seen he sent me an image of White Nose. It was where it always was and had tolerated a camera being poked into its lair right in front of its face.
Another example is the mighty river Trent that I have barbel fished a lot. The few repeat captures I have suffered have always come from the same swims as I have caught them from before, despite fishing swims either side. Some of these repeats have been a few years apart too.
So, it is quite possible their cousin's on the Clyde will hang around where your friend has found them for quite some time.