Toby:
When you printed the parts, what layer thickness did you use? What sort of tolerances were you using between parts?
I normally print at 0.2mm layer thickness. This is my compromise between print quality and print time. I've had to do a lot of playing around with my printed parts but I've found the following tolerances to work for me and my printer:
Less than +0.1mm gives me a very "tight" press fit, almost impossible to use. Once you mash the parts together, if you can, they are NOT coming apart without a fight.
+0.1mm to +0.2mm will give me a press fit, with +0.2 being a "light" press fit. The parts will go together with finger pressure and they will come apart without an argument.
I use +0.3mm and above for parts that I want to fit together and move.
3D printing, at least the FDM/FFM processes normally used, is kind of a "sloppy" procedure. Printing in layers as we do, this can't be helped. You CAN'T print an absolutely flat side no matter how hard you try. Under magnification it's still going to look like the side of a log cabin. It can be minimized, but it can't be eliminated.
Hope this helps.
Don