I bought the medium size specifically for my B17 flyer that's on my main commuter bike. I also tried it on my Brompton which has the B17 standard. It fits both. I've got it attached to the under spring with the velcro and can be stuffed in under the saddle when not in use, so it's always with me. The bike's been sat out in the rain a couple of times and it does the job - dry seat. I like the idea of no logo on the cover - discretion!
PS note: this cover is for protecting the saddle from the elements when NOT riding the bike. You take it off when cycling.
Edit (6 months on): I've since bought another for my wife's B67S saddle. The medium size does NOT fit the B67s and so in this instance I bought her the large size which fits. I conclude that concerning Brook's city and heavy duty range of saddles to go for the large size.