I have a Fluval U2 fitted to a 60(ish)L tank that's currently cycling. I think that the flow is too strong. I know it's probably a little over rated, but the filter that came with it is hopeless, and find the Fluval U series to be in high regard.
I've turned it down using the built in adjuster but it still creates a small whirlpool! Would I be able to fit a spray bar to it?

I made a DIY spray bar once out of a bit of syphon tube. As long as it doesn't slow the flow too much then it should be OK. Which way is the outlet pointing? Just changing the direction a bit might help.

With the syphon tube did you just prick holes into it and DIY attach it on?
At the moment it's on the right hand side of the tank face the left hand side, so along the full length of the tank.

Try turning the outlet slightly to the right so it points more towards the back wall of the tank. That should break the flow slightly and calm things down.