Thanks Jipe - I've not yet concluded the mystery of the measurement that brompton refers to, the current seatpost seems to be around 550mm overall length.
I did measure approx. 520mm from the base to the underside of pentaclip, so it's more than 520 overall.

I also measured from the top face of the seat clamp to the topmost tip of the post (flush with top of pentaclip) and x = 470mm

based on this my preferred distance x is 440mm.

I guess if I order a chinese Ti seat post that's 550mm I can't be far off the mark...

It's anyone's guess have I got a 550mm or a 520mm post :-) I'm guessing it's what brompton call a 520mm.