On the H.Avon it is 99% clean gravel,with odd clay boulders,where you used to be able to see that gravel easily ten feet deep,I have never myself left a bait two hours,but I understand the logic,when numbers of barbel were greater more regular casting was key,but as numbers dwindle they really are spooky,it is a case of what works for you,maybe if a back lead wasn't used the chub would be more 'aware' and be less of a nuisance,however the H.Avon is a challenge all of it's own,you don't say how big these bait robbing chub are Steve,but knowing the river these days they are liable to be some big buggers doing you over,lol.