Bike.1 (a 26 inch wheel Haro) is giving an (very) intermittent problem, which I think might be a loose connection on one of the Halls. I've ordered a sensorless controller; when that arrives, I'll fit it, and if that cures the problem, great.
Bike.2 is the other Haro. I was going to take it out tomorrow, but after I'd installed a voltmeter, and a cable shield (to protect the wiring as it enters the motor), I took it out for a test. And towards the end of that test, it was running very roughly. I think something is wrong inside the motor.
Bike.3 is an Everest folder 26" wheel, which I converted to electric. It uses a sensorless motor, which means that it's not good at starting from stationary, but it's OK if I kick off a bit.
Bike.4 is the old Synergie, which I stopped using after the bottom bracket became very worn. But I was able to replace it, and now it's working. But it has a 20 inch wheel, which isn't as good as 26 on rough ground. It needs a speedometer, but that's easy to fit. The controller doesn't like running on 8s unless they're newly charged, so it'll have to be run on 10s.
Bike.5 is the downtube, a brushed motor. It's the lightest bike, and has a very handy center stand, but when I run it, the motor gets hot after a while. And when it's hot, it runs *very* roughly. So I can use it for caching, but it isn't ideal.
Bike.6 is the Forza. It's non-folding, so not too good for caching.But otherwise, it's a great bike, and I use it for local trips.
So after thinking about all these for a while, I've come to the following conclusions.
1. I'll use bike.3 for caching, for now.
2. I need to buy a new motor.
Tomorrow I'm going near Peterborough; there's a circuit called "Burnt up bike ride" which I can't resist, and then I'll go into Wittlesey and environs, and mop up what I can find there.
... later ...
I've rummaged around the internet a bit, and I think I might get a Mac motor from em3ev, which I can get in a 12-T winding. That means, less speed, but more torque. Which means that my top speed will be 15 mph, and I'm OK with that, but it'll be very good at helping me get up steep hills, or across rough ground that's also uphill, a very common situation. It's looking like around £210, plus £55 shipping. Or there's a company in Portsmouth that can supply a similar wheel, for £180, including shipping. But they don't tell you anything about the winding, and I suspect that, unlike em3ev, they don't actually know anything about what they're selling.
Some thought is required ...