Awww, karmgirl, this brings back horrible memories for me from last year when I lived with two of my friends. When we moved in initially they wanted to share the shopping bill, but I insisted that I wanted my own as they both earn a lot more than I do (so chuck any old thing in the trolley) and one of them drank like a fish.
One of the girls started off asking to "borrow" a bit of my milk, a slice of my bread, which I was fine with, but it got to the stage where I'd go to the fridge and have no milk left from two pints and it really started to get to me as she would never replace it. It felt really petty, but it was more that I needed to know that my food would be there when I expected it to be instead of me having to go without out.
And breathe! :) My best mate and I moved out not on the best terms with this girl, anyway for various reasons, but everything that bugged us all about each other came out in a screaming match on the last day as we loaded our cars. Funny looking back on it, anyway.
Don't not say anything, hun, or it could all come spilling out when there's a bit of butter missing... :)