Thanks chaps,
I’m aware of the c of g and ballast aspects, but as has been said, assuming ‘type b’ is flown with the same seating standards as ‘type a’ will catch you out. But good to get things I wasn’t aware of, like:

Flying from the rear cockpit also made it easier to detect/become aware of ‘Swing’ during take off and landing !

The critical aspect (why I was asking, and thanks, you’ve answered this!) was if a 30s pilot was taking a friend for a fly in a DH.60 (or a Avian) the friend would be in front. If the friend was going to be (or was) learning to fly, they will be in the back. So different seats depending on the intent of the flight.

Thanks again,