I'm sure the 5 speed Sturmey Archer hub below will fit, as it was previously used on the T series of Bromptons. There seems to be a new one going cheaply below if you act quickly. A 5 speed would give you ample range. Could be a real bargain modification:

http://cgi.ebay.co.uk/ws/eBayISAPI.d...tchlink:top:uk