Ok after lots and lots of thinking I came up with this , simple and uses the existing seat sub-frame and the minimal amount of extra parts ?
This is probably the maximum tilt as the seat will hit the frame.
The front is a large door hinge held around the frame with 2 x 8mm bolts and some angle iron. I do think angle iron sort of screams of a diy bodge however I chose it because it stops the plate bending to easily and with the vertical surface pointing forwards it will help keep the nut's clean.
It is square I think I have botched the original seat mount and it is now sat at an angle.
So this is what it looks like when sat on the frame. I doubt I will be using the Q/R's as they are only screwed halfway into the Q/R nuts and I think that is not safe. On the clamp using the Q/R's I have freed the vertical tab on the other side, it was welded with some cardboard spacers as advised by Sandman to gain clearance for the paint so I carefully cut into the cavity made by the spacers and released the tab , now it can't get bent under the frame and then lock the whole thing up as it has done in the past as undoing the top fixing makes it come free from the main frame.
So this is only prototype wrong length bolts no lock nuts and no paint , Need to get the seat bolted to the sub-frame then I can put it on the trike and give it a test......
Anyone can build to a plan. After all, it is someone else's proven dimensions and methods. It takes creativity to suck-it-and-see so that it too, in time, can become someone else's plan. Along the way, there is a collection of scratches, errors, changes, bodges, discards, etc. Those that stick at it, reach its final conclusion - even if that conclusion changes to proving that it can't be done.
It's the journey that is important, not the destination. As an example, I'm concentrating more on the journey of life - not the destination.
Yes ' cold enough to freeze the balls off a brass monkey ' [ look it up it is NOT rude ! ] here yesterday however I got the new seat frame pivot/clamp assembled on a piece of scrap frame material and then transferred it to the trike , only 4 bolts and 2 zip ties however a real chore in the cold and high winds ? hiding in the back of the garage where there is no light ...
Not tried the tilting in-situ yet as rear rack needs removing and box assembling and I had had enough and went inside to thaw out...