Richard,
I have some experience(?) of targetting bigger Barbel on the very river you speak of....
....I used to fish with 2 rods and only 1 bait. Bit of a mad idea you may think - certainly sounds like one that's for sure!!!
This was the rationale behind the "method"
1. The upstream rod was "fished" with just a feeder, no hooklink and bait, and was regularly re-cast as in the usual fishing scenario. This regular re-casting (every 10 minutes) laid down a regular trail of bait and, in my warped mind, replicated exactly what the Barbel were used to, except of course there was no chance of them being spooked by a hooklink and the occasional disturbance of a hooked fish. This would surely get them switched onto the bait.
2. I surmised that the smaller Barbel, Bream and Chub would be onto the bait quickly and would attract the bigger, more wary fish to come in for a peek at what was going on. In my minds eye, these bigger fish would hang back a bit and maybe pick off the odd morsel that got past the smaller ones.
3. That brings me to the downstream road....this was fished with a simple lead arrangement and a long hooklink baited with a small'ish hook and just a scrap of bait - to replicate the scraps of bait that might just get missed by the small fish upstream and trundle into the view of the bigger ones? This rod was cast and left.....until it was time to go home or it was ripped off the rest - whichever came first.
I once caught a syndicate best Barbel a few seasons back doing just this....2 rods - 1 bait
More food for thought???
Paul