This should be a silly question, as you said "same size and spec" but have you checked the width, even though you ordered the right one?

You probably know that Brompton "highly recommend" changing the sprockets with the chain. The sprockets are cheap (in Brompton terms anyway!). It's not blindlingly obvious to me either why that should be necessary or that the chain should come off, but there it is.

http://www.brompton.co.uk/extranet/d...f/ds-chain.pdf

http://brilliantbikes.co.uk/56-bromp...ts-and-spacers