i'm pretty sure this isn't what you're asking as it's futures only... but will dump it here in case someone else searching for
open interest finds it useful.
http://liffe.com/data/dfut090416.txt
FTSE 100 future is 3/4 way down, open interest is far right column, it gives the OI change to previous day as well.
A few years back I made a VB program that chops the file up and strips out the crap columns (it's designed for stirs but easily adaptable). I'd be happy to share the project files, just pm me