An O-ring from around the top of your seatpost, just below the saddle?
That would be one of these, which seems likely. Ta!
https://brilliantbikes.co.uk/brompton-handlebars-and-seat-posts/2764-brompton-replacement-seat-post-o-rings-pair.html
@Bainbridge started
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An O-ring from around the top of your seatpost, just below the saddle?