Hi Joe,
Some good advice there from home rollers so far. I was never rich enough to experiment at want with high quality mixes making the most of the stuff that was available to me when I worked at an animal feed manufacturer in the nineties. Before then, my main home rolled stuff was semolina and soya flour with a funky flavour. These did catch some fish but were **** except in deep water; I'm talking carp here, not barbel.
There were though some useful ingredients there such as CeDe, lambs and calfs milk powder, stacks of Provimi 66, cod liver oil (I used Codlivine which added fibre too), salmon fry crumb, garlic powder and molasses to name a few.
From my experiences with carp fishing, obviously I am making the assumption that carp and barbel are too similar to warrant thinking out of the box, I would suggest buying a proven milk base mix, if one really does exist, and add one other dry ingredient to it to create the triggering response. Once you get some results, of course if you don't, then change the lone addition for something else and add either more or less until your getting more bites.
It's quite simple really. I'm almost wishing that I had the time to experiment on my local stretch. Luckily, I don't have the time!