What would be a good lens for a canon 40d? I dont really want to spend more than £400 but could do £500 if needed! Preferably with IS. Currently have a 50mm which isn't suitable!
I would go for the 55-250mm, it'll be slightly more versatile. The difference between an f2.8 lens and an f4 lens is the aperture the lens will open to - basically the smaller the F number, the bigger the aperture. That lets more light in, so the amount of time the shutter needs to be open is decreased. If you want sharp pictures of a galloping horse on a dull day, keep saving for an f2.8. If you're happy to stick to bright days or slower paces or don't mind putting the ISO up to compensate, then an f4 will be fine.
The only thing against the 55-250mm is that it's an EF-S lens - this will be fine to use on your camera, but if you ever decide that you want a full frame body, e.g. the 5D, 5D Mk II or some of the 1D series, then it won't be compatible (the EF-S series is designed to only work with crop sensors).