Hi Pete,
This is a method i used to use a lot for Barbel, using old film canisters for the feeder, ( Scottish blood in me !

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I always hair rigged using a size 14 crammed with up to 25 maggots which will prevent the 14 hooking the Barbel to a size 6 drennan specialist or ESP Raptor G4, naturally the size 14 must be barbed or you'll lose quite a few maggots.
The 'nuisance' fish in my opinion aren't a nuisance at all, any Barbel in the vicinity wont ignore the frenzy, and in my opinion is what draws them to your bait, the huge ball of maggots will prevent anything small getting it in their mouths except probably larger roach, dace or bream, and the first two i wouldn't mind catching anyway, though i never have on the method.
Likewise Chub, with the exception of one 5lber, i have never had another on this method, they seem not to like large bunches of maggots, and i've no idea why. Also Perch though i've caught on the float with large bunches of maggots, never with the hair rigged bunch on a feeder, but it's probably more the time of day with perch.
1/2 a Doz maggots is no good at all you should be looking to get at least 20 better 25 on the 14 if you've got the patience !!
Your rod tip will bang, and twitch away, but believe me as long as you have a large enough bunch of maggots on the little stuff wont be able to suck them, eventually barbel in the vicinity just can't help themselves seeing the frenzy.
I've also used this hair rig method static with a lead instead of feeder, but you must be able to accuratly feed your spot making sure the maggots hit the bottom in the immediate vicinity of your hook bait.
Feeder or lead there must be no let up in the intoduction of maggots, heavy to start with, then slow it up, but no more than a few minutes before the spot sees more maggots, and idealy so you don't have to keep recasting, as with the lead it's better if you can accuratly feed by hand or catty, and with that in mind i always take about 3 pints with me.
The only down side is - for a productive days Barbel fishing you pretty much have to know that barbel are present, i don't think it's that good for pulling barbel from very far, the maggots just wont get that far, if you think that would be the case soak them in a fish oil, before putting them in the feeder.
It can be fantastic sport though if they are close by, and the bites seem that much more Savage ! so stay close to your rod.
Good luck Pete.
Ian.