Well, I got to the bottom of why my little chickens look so different: they are not pure. The guy bought Java 6 eggs from a breeder in Belgium (who's contact details I'm trying to get now). He brought them here, hatched them and crossed them to his other bantams (unknown breeds). The chickens I bought were the result of that X breeding. I wish he'd kept them pure!
I am going to try and get some pure Java hatching eggs, but I've had very little joy so far in finding a breeder. Hopefully I'll be able to get hold of the guy in Belgium.
Thank you for telling me more about this breed and for all the links and info.