Firstly Jez, I'd say for such photos, f20 is overkill. Using
Online DOF Calculator, shooting at 50mm, f22 at a distance of 6ft, you have a total of 4.5ft of the scene in focus. Going up to f8 still gives you 1.5ft of scene focus, so I'd imagine more than enough to cover the depth of your subjects.
The other thing I notice is that you are using pretty regular/uniform backdrops for the shots which does, to an extent, reduce the need to throw the background out of focus, as it's not really that distracting as is. Having a shallower depth of field could still help blur the detail of the dado rail and such though, focusing attention on the subject more.
So, a 50mm f1.8 lens could still be an option, but to see if you can get what you're after with what you already have, try using a wider aperture for a start. You could try a series of shots at different apertures to see which ones work for DOF and help keep the shutter speed up (if you're hand holding).
More generally, try using a really bland backdrop that contrasts a bit more with the subject. Personally I think the colour of the wall in the C-3PO shot is a bit too close to the colour of the subject, though this is also true for the Clone Trooper, but it doesn't seem as much of an issue. Try black for a start though it
may be that this means you have to keep the subject further away from the backdrop than you do currently as it will 'suck' some of the fill light from your subject due to the lack of reflection. Having a greater distance between subject and background will help throw the back out of focus though which isn't a bad thing as discussed. Oh, and I don't mean to mega picky, but try and keep things straight. The Clone Trooper helmet shot has a good straight horizontal between the white and grey of the backdrop, but the subject itself isn't vertically straight and personally I keep being drawn to that. Having a one colour backdrop will mean you can shoot with more subject bias for such things.
Anyway. Are these things you build yourself? Collect? or trade?
James