Thanks to the informative and inspiring information on Rib.Net i have started doing this Transom Replacement (Comments very welcome)
You can see how rotten the core is, I used a hair dryer (rather than a hot air gun...) to release the glue bonding the tube the the sides of the transom, thought i would have less chance of setting everything on fire
Chipping the core out and blunting all my chisels
All the wood is out down to deck level where it seems dry and strong
New transom cut from 18mm 8 core marine ply, the gaps at one end are to give it wriggle room to slide in, ill epoxy fill these.
Now to Epoxy prime it and set it in with more epoxy. need to go and get 10 clamps, advice is not o over clamp and squeeze all the epoxy out.