Montague option is fine.

I have seen Dahon Jetstreams' on ebay ,(new I think), for £450. I can not think of a better bike for the money and purpose that that.
Currently £600 seems best price,
http://cgi.ebay.co.uk/NEW-2009-DAHON...#ht_8560wt_967
but wait and I am sure they will come around again for sub £500.

http://www.winstanleysbikes.co.uk/pr...ding_Bike_2009

Rultland cycles might be worth a try also.