so i finally worked out how to do this. i bought a stock 34.9 -> 31.6mm aluminium shim. it's much better quality and much longer than the plastic PoC shim that comes with the brompton. The 34.9 shim was slightly too small for the frame but with 2 layers of drinks can it fits perfectly (with a bit of help from rubber mallet). so now i have my aluminium seatpost and railed saddle without that horrible steel post and pentaclip and without the ridiculously priced bromptification Ti post. hope this is useful