Did a little more, things move slowly, or at least look that way. I've done some tests on floor strength etc in preparation for mounting the spare, it's pretty heavy.
The frame at the base of the rear bulkhead screws through the floor and into a second piece underneath to stop the floor from flexing. I'll be adding a second piece underneath the other side of the tyre winch and will then mount a piece of angle where my temporary piece is and extended it out to the second set of bolts and across to the area where the second underfloor strengthener will go.
With all this the floor can take my 25kg spare

and not flex or rip apart. Anyway, here's some pics of where it is so far. It shows the temporary steel as a test of it's impact on floor strength.
Base of rear bulkhead, sleeping area side.
Base of rear bulkhead, kitchen side, with test steel piece prior to proper angle and extension to pickup second set of bolts.
And underneath, you can see the chain winch hanging through the spare. I'll also be adding some more frame under here to pull the tyre up against. The unpainted piece is right below the bulkhead and is screwed into by the lower bulkhead framing.
Winch
The winch handle will extend all the way out the side of the frame with a sunken bolt head that is the same size as the stabilizer winders so I can use the same handle to raise and lower the tyre as the legs
Cheers
Steve