Up on eBay now is a very nice original Civil Defence beret. It's the type with no inner headband. It's made by the British Beret Basque Ltd and was introduced in late 1941 (along with the other battledress and serge uniforms). These are getting extremely hard to find and a size large is perfect for the modern day re-enactor.
UPDATE: This may actually be a post-war example as it does not have the correct ARP pattern number printed inside but has a CD plus something else missing. There is a chance the 4 was originally a 5.
News about interesting insignia, ARP related info and period photos that turn up.