Can’t remove the telescopic seat post for my Brompton? Any ideas how

I’ve removed the saddle and and the telescopic extender bit. I’ve left with the seatpost pole which has a clamp thing at the top (circled in pic). The clamp thing stops the seatpost sliding out at the bottom.

I want to take out the whole seatpost. Any ideas if I’m doing it right and how to remove the clamp thing?

Thanks!

by Junior_Persimmon_242