With so much going on I have had little time to devote to the cars but finally I have ordered some wood to repair the seat with.
It seems that the consensus is that Beech is the wood used for the seat in our Austin 10 while Ash is used for the various parts of the body framework. I have been told that Beech despite being a hardwood is well suited to use in seats as it is also quite flexible. In the frame of the seat we have a bit of spring to allow for the bumps and other shocks that the seat will have to endure. That kind of makes sense to my mind so I have ordered a couple of lengths of 3/4" thick Beech. I intend to build a new base section first and see if I can improve the design slightly to ad more strength to it perhaps by using a couple of bolts to hold the metal bracket in place.