My B40 has a C15 centre panel ( the one between the oil tank and the tool box) which has the original air filter welded in place.
I understand that complete ones are quite hard to find. I don't intend to use the original filter, so if anyone has a decent b40 centre panel ( exactly the same but without the filter) I'd be happy to do an exchange, otherwise i'll just remove the filter, which would seem a shame.