Managed to play today with the aid of Marcus. I'd spent a few evenings with PD bending up some tube to make a new subframe and getting the general shape done. I then whizzed it to Marcus's for full welding and making some seat mounts.
The seat itself was quite strong and fairly rigid, but I made up a plate to strengthen it underneath and allow us to bolt the seat down to the subframe. Once riveted on it made it super strong and I was glad made it. 
We also made some tabs  for the seat to bolt through to and the rear mount for the tank. Once all bolted up it was absolutely solid and has a great position as well as being aesthetically pleasing.

Project cheap, free and steezy
Really glad with how the lines flow - the line of the bottom of the tank follows the bottom of the seat and the top of the rear of the seat is in line with the top of the tank. 
Here you can see the plate underneath

Project cheap, free and steezy

Project cheap, free and steezy
And Marcus trying it out for size

Project cheap, free and steezy
Now to get the engine in.