hi ade - i know how frustrating it is mate. i went from October until january before i caught, despite going once or twice a week to a local venue - even now i have only had five out, the biggest being 18.2lb.
i was trying similar tactics to you, then tried a total change - maize. it is clearly the key for me water, but it may work for you. i would suggest firing a few cattys worth out over a 5 m squared area or so, alonside some cooked hemp, before you have even set up your rods. also, make up a groundbait using 1 x loaf of white with crusts off (stick in the food processor) and add 500kg of crushed hemp and a 1kg bag of the new fennzied hemp groundbait, stick in some maize and a keep it fairly dry, you cen then ball a few out, plus put a small amount in pva for your cast.
the best results i have had have been on 1 large single grain of maize, alongside a small plastic grain (to balance it up), and a size 10 wide gape. i use a fairly long hooklength of about 8 inches, using 10lb korda iq xtra soft.
if you give it a try, let me know how you go
like you, i had tried everything, snowmen, pop ups, large boilie, small boilie, chopped boilie, trimmed boilie, pellet, maggot (did get a tench), worm (got 2 bream)
keep it simple, small and as light as possible
hope it works for you