Thanks Bob.

A quick check of Sheldons site suggests that my fixed is running 74.5", and my Brompton is running 43.0/57.4/76.5 which makes sense. I spin out too quickly on flat in middle gear, but can rarely get into top gear unless there's a downhill. I could (I think) go to 54 x 13 on the brompton, which would give 50.1/66.8/89.0, which might help a bit with the middle gear, but it might still be too low, and the top gear I think would be pretty much useless. 50 x 15 would give me 40.2/53.6/71.4, so I could probably use top gear on the flat instead. I wouldn't have anything extra for downhill sections though, and in London I can hardly imagine using the lowest gear, but I guess it might have some use if I ever go somewhere hilly.

Am I right that the biggest I can easily go at the front would be 54 with brompton cranks? And that the rear sprocket can be 13-15 on the SRF3?

Has anyone got any thoughts on what might be the best option? What gearing are others running on their Brompton?