Yeah i've tried a few. I think I might have it sorted though.
Placed an order for 2 sets of these.
http://www.amazon.co.uk/Repair-Spiral-Pinion-Makita-4100NH/dp/B00EZCQO7W/ref=lh_ni_t?ie=UTF8&psc=1&smid=A50DZI580G3JXIf it works out it'll be very cheap! Although the ratio is a bit low, the rear wheel will be turning at about 370rpm if the motor is going at 6000rpm. That'd be close to 30mph and might not give me alot of torque, but i've really got no frame of reference on how much is required and how much the motor supplies.
Here's the plan so far though.

There'll be a chain connecting the gearbox to the wheel's sprocket. It's a bit of a crazy plan, gripping a sprocket to the center hub (opposite to the existing sprockets) of the rear wheel via an aluminium block.
The things I'm concerned about is if the gearbox and motor will fit well in that small space between the frame and the wheel (I don't want to put it too far forwards in case it hits my legs as I pedal) and if there'll actually be enough of a gap to feed the chain through. I might need to make some kind of idler to push it down and out of the way maybe.
If it works out it should be a pretty good system though. The two main types of DIY electric bikes i've seen either turn the rear wheel via friction on the outside (which seems like it would have alot of problems), or turn the pedals (which would be tiring to keep up with, but I guess that's more of a power assist for steep hills). The free wheel I bought should ratchet if I pedal without the motor, so I won't be fighting against a gearbox and motor when purely peddling.
Boy i'm really using alot of weasel words. I'll honestly be surprised if this works out first try, but i'm having fun non the less!