The combined 12/18 sprocket was supplied by Highpath Engineering (their website at http://www.highpath.net/cycles/index.html is very educational, but doesn't actually mention this 12/18 sprocket) and cost 39 pounds sterling.

I spent almost 2 days installing this sprocket set, filing the end of the
chainstay, testing and taking the rear wheel off and back on quite a few times, but eventually gave up getting a clean gear change. The chain would 'ride' on the top of the teeth 18T sprocket, and immediately jam between this and the end of the seat stay. This happened both when testing in my workshop, and when out riding.

However Chris Bell at Highpath Engineering has been very helpful on the phone,
and has offered to grind down the teeth on the 18T sprocket a bit, and I'll have one last try when he has done this. I have noticed that the standard 15T Brompton sprocket has some teeth which are shorter than the others, and I wonder if this is what Chris has in mind?

I'll let you know how I get on, but in the meantime, I'd be very interested to
hear from any others in this newsgroup who have successively installed this sprocket set!

I don't know whether Highpath produce 12/17 or 13/18 versions of the sprocket
set (or whether I'd have the patience to try them if I can't get the 12/18 to work!)