I know there is one online somewhere, 'cos I've read it.
I've gone over the 1gb limit, and it's a soft limit, I think. You get warnings, but at least up to the extra I went over, I wasn't charged or cut off.
Still can't recommend Three. The packet data connect from whatever part of the network allows me to connect, and both my Three phones report it as a strong connection, but for the last week, no data is being transferred (and no, I didn't go over the limit
).