Thanks,
I'll see if I can find a couple of washers
I guess another thing I could do is to shorten the chain by a couple of links (it's due a replacement soonish)... if I no longer use the 16T sprocket, I should be able to live with a shorter chain and therefore more tension/fewer chances of things going wrong. Fold worked fine when in the large sprocket, so I can't see any drawback...
On a related note, do I need a Brompton specific chain, or any 8-9 speed Shimano chain would work equally well? The 16 T sprocket did look like a Cassette one, but the 12 T seems quite a bit thicker...