Isn't this more likely the sleeve needs replacing https://www.brompton.com/support/knowledge/seatpost-sleeve-replacement
The clamp is loose, not tight so I assumed it was the clamp itself, which looked like it had play in it. Might have to book it into Compton Cycles for a check over.
@andyp started
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Isn't this more likely the sleeve needs replacing
https://www.brompton.com/support/knowledge/seatpost-sleeve-replacement