@NordicCnc

Thanks for that tip re the Step files. I went back in and re-downloaded all the files as 'Step' and my problems went away.

The only problem with the rails is that you need to specify a length before the website will create the 3D file for you. Think I'll knock up my own version and just keep that as a 2D drawing that can be extruded to the length required. Although it will obviously lack the actual positioning of the mounting holes.

Cheers