A search of the Internet reveals that Caradice are bringing out a new large front bag, which appears to be Brompton specific.
[URL="http://https://www.secured-url.co.uk/carradiceshop/ccp51/cgi-bin/cp-app.cgi?&pg=prod&ref=brompton-bureau"
It is expensive at £70, but looks to be of good quality as most Caradice bags. But I could buy 14 rucksacks (see earlier post) for that price!
A word of warning on Caradice bags though. Their "Rackbox", a carrier mounted rear bag is a good idea, fitting well on folding bike carriers, but a poor design, having inaccessible poor quality zips and an internal pocket so small that it is useless. They now have an updated version which looks to be much improved. Don't buy the earlier model.