Thanks for the advice everyone! I'll have to put some of those things on the back burner for a little while however, as I've just discovered a somewhat horrendous issue with my drive wheel bearings!
I won't post a picture (it's dark outside so it'd come out badly anyway) but the main bearing on the axle on that side has disintegrated, some of the needles have embedded themselves in the plastic mount and there's a *lot* of melted plastic.
However, I think I can do something to fix it!
If I get three new bearings, one for the drive wheel (one directional, obviously), one to fit in the right hand side of the section of the motor/axle mount and one to fit where the "integral" bearing was (which has done much destruction) with some sort of collar to fit around the damaged sections (perhaps glued in place?) then everything should be ok.
Finding bearings... Well I'm not sure about that. I found 3 places near me (Cambridge) where I might be able to get some to fit. I'll have to see what I can get!
If anyone's interested, I got out my callipers and measured that I need these sizes:
Firstly the single direction one, 32mm external diam. and 20mm internal. ~26mm long. Second and third are both free to go either way, one 22mm internal and 32mm external (~12mm long) and one 23mm external and the internal matters less, but as long as I can fit a 14mm internal diameter tube into it it'll be fine.
I'm hoping to get some off-the-shelf from somewhere...