After much thought and comparison, I've ordered a sensorless infineon controller with 6 mosfets, which means it should be able to give my motor up to 25 amps. A mosfet is a metal–oxide–semiconductor field-effect transistor. They're very good for high-power applications, such as electric bikes. And ... the controller is programmable! So I can program in things like top speed, high and low voltage cutoffs and current draw. Actually, I ordered the sensored controller by mistake, but I emailed them and said I actually wanted the sensorless, which is the same price (I suspect it's the same device with fewer wires).
I've also ordered the cable for programming it, a neat thumb throttle, another torque arm (see previous blog for what that is) and a nice box for testing ebike motors and controllers. From China (is anything made anywhere else now) so it'll take a few weeks. I'm planning to install it in bike.3.
Total cost, including shipping, is less than a tankful of petrol for the car. Just to put it into perspective.