I have an Ultimate Fun Fly which flies very well with the exception of inverted.
When I fly inverted, the plane immediately wants to roll out of level flight. I have to contanstly battle the airelons to try and stay straight.
Does anybody have any ideas why this is happening, or is it the design of the plane?

Hmmm. Mine flies good both ways ... but I have mine on the tail heavy side. Other than that, it should fly the same inverted as it does wheels down with the addition of any elevator needed to fly level.

Sounds to me like you have the incidence wrong on the top wing. Dennis