if you didnt spend so much time fishing for them and instead read the threads on whisker fishing and learning about how to do it you might catch one
take it from one who knows about barbel fishing [ive caught one from a still water and it went 4 oz]lol
have you tried goi ng for a mixed session as opposed to all out barbel with massive rods, huge baits and major tackle?
spoke to a lad off here who showed me some pics from his pleasure session last week on some river and he went out for the Bream, fishing worm over groundbait and hemp, and had several barbel pull his rod round through the day, welcome fish he said
not sure what tactics your using but if you have a huge cage and 22mm halibuts, maybe it isnt working, try something else or try and master the river to start with as opposed to the river and barbel at the same time