This should have been plain sailing - I ran into an unexpected issue - the masking tape I had put on the chassis tubes for marking out some weeks ago had permanently attached itself and proved difficult to remove. White spirit easily desolves the glue however the masking tape is waterproof so needed scraping off first before the white spirit could clean up.
The planned rivets worked well for holding everything in place:
This row on the underside just needed the ratchet strap to hold things in place long enough to rivet, then the rivets hold the side in tight for the bond to go off. Its not very clear from this picture - but I put a line of 5 rivets on the left hand edge of the floor which holds the outside (right) edge tight against the glued chassis tube:
A little heath robinson to hold the panel against the chassis tubes, and the rear quarter installed over the top; taking care to only place rivets where they will be hidden by the wheel arch later:
Next step is to put the front suspension back on & install the front brakes, then I can move the car over to the other side of the garage and repeat the exercise on the offside.