I thought that I would share a method I have been using for the last 22 months to set the hight of a Brompton seat.
What you need is a sheet of plastic motorcycle tank protection film.
I bought mine from Hein Gericke but I’m sure that there are other sources of this self adhesive plastic film. It is about 1mm thick and quite hard wearing (I have only needed to change this film once)
What you need to do is ride around for a while until you have determined the best seat hight for you. Then take a marker pen and mark where the seat post enters the frame.
Now pull the seat post fully up and measure the distance between the pen mark and the frame (in my case it was a little over 1”). Now with the bike still in the ready to ride sate (e.g. the rear wheel NOT folded under in the “park” position), drop the seat post down to the floor to expose the bottom of the seat post. Next cut a strip of plastic that corresponds to the measurement you took between the pen mark with the seat at your comfortable riding height and the fully extended height. Wrap the strip around the bottom of the post, mark where it overlaps and trim it to length so that it forms a collar. Remove the backing paper and wrap the protection film around the bottom of the post just above the flared section.
Now each time you use the bike you can just pull the seat up as far as it will go and it will stop at your perfect height. I have been using this now for nearly 2 years and it works great and as I say, I have only had to replace the film once. It is also a simple task to remove the film if required.
Here is a photo of my Brompton with the film fitted.
takes longer to explain the method than to actually cut and fit the film.
I hope this is of use to other Brompton owners.