Loads more to think about, thanks. In short, the consensus seems to be that there's not much off-the-shelf that will glide along up to 25 km/h and ride like a non e-bike above that.
The discussion on battery life was useful, although I hope that's not the norm. Whilst I'd like a longer battery life than 3 years, I'd not be devastated if it gave up at that point--by then I'd know if I was going to use the bike regularly and what I really wanted from one.
I'm reluctant to go down the conversion route due to the time and knowledge required as this would turn it into much more of A Project and I'm sure to get something that didn't fit or wasn't compatible. Whilst I'm reasonably good at technical stuff, I have pretty much no experience of modern bike maintenance--I had a good look at my current bike, and the some of the teeth on the chainring look quite worn--so it probably needs replaced. Furthermore, it's always made a slight clunking sound in top gear--the front derailer seems to rub against the chain when I put push down (but not when turning the pedals by hand off the ground), and fiddling with the indexing doesn't seem to fix it. The chain's never been replaced either and the bikes about 6 years old (though seldom used). In short--maybe 15 years ago before I had kids, I'd have taken this on, but I don't want to add another project to my list of things I'll never get round to...
I think I need to take some test rides to see how things feel--the nearest dedicated ebike shop is about 50 miles from me, but I'll try and nip up over the next few weeks. Also gives me time to see if I can get work to sign up to a cycle to work scheme.
Faro and Metroneer look good, I'll keep them in mind. I'm also reconsidering the Camino--anyone any experience of its motor drag? Also, is it really legal for it to have a throttle that works without continuously pedalling?