Got around to changing seat and adding the backrest.
Did 50 miles each way, to London Motorcycle Museum Triumph Day last Saturday.
The backrest was set on Number 1, it's still just a tad too far forward, but it feels a lot better on base of spine.
The seat itself did not feel much different to original seat without the backrest hole.
Have to do more miles to see just how much better this will work out.
Will have to get the backrest to raise a little higher, its fully extended upwards, will have to extend the slot a little.
When I get the time, I will make up a new spin wheel with lower adjusting steps on it, to get the arm to go back a little further, or maybe remove a little metal from the arm itself, it could do with a little more rearward adjustment.
So far so good.