me and my friend too a shit load of food and drink in with us last year, of course this was fucking stupid thing to do, as the drink leaked and mixed with the food and before i knew it my bag was filled with sludge, more to my suprise i'd left my mobile in there, i weren't best pleased i can tell u
...the reason you can't take your own alcohol is only because of the packaging it's in: cans or glass - both could be weapons. Well and because they wanna take all your money in the Carling tent LOL
If you want to sneak a bottle of water in just stick it between your stomach and your trousers (waistline) and make sure your shirt is covering it. Sorted.
...on who's checking. Some are more diligent ( and jobs-worth) than others. Also might depend on whether a supervisor is nearby. I've had booze and food refused entry. Even with beer bought from the bar just outside the arena - I was told to drink it or throw it away before coming in. Like I was going to throw it away! Glug glug glug...