OK - window is out - took 40mins - it's 4mm glass, not 5mm.
I managed to save the piece of trim vital to the project - that being the curved piece at the door edge - and it's in reasonably good shape for the re-install of the acrylic.
The long top piece is not so good, but I believe I have enough of it to recreate a pretty good copy from readily available window sealer's, along maybe with some liquid rubber and/or a little bumper repair mastic.
For anyone contemplating this job for whatever reason - I just used a small steak knife, some wooden wedges and about 10 sharpie's. And I simply placed the blade between the glass and the chassis and kept scoring the mastic until I was through, and kept going all down the door edge, and then along the top curved portion. The corners required more patience and power to separate the mastic - this is where the steak knife was pretty essential. lol When I had completed the above, I was able to wedge the window as far out as the top bodywork would allow me. Then it was a case of getting inside the car and again running the blade along the glass/chassis join to separate the mastic. Once the two bottom 'corners' were free - I cut off the three base clips with the sharpie and gently eased the glass out. Job done. I don't think this is possible without damaging some of the rubber to the point of no-return so bear that in mind before you start.
Right - waiting for the 4mm acrylic and the fuel filler neck to arrive so that I can move on to the next stage.
I want to try and recess the neck slightly into the 'glass' - so current plan is to make a 6mm copy of the original window in acrylic and correctly heat shape it so it is a carbon copy of the original glass. then cut a 5" hole in the 6mm where the filler neck is going, I will then mount the filler neck into the 4mm acrylic and place it in the oven at about 100 degrees. I then hope to remove the 4mm place it over the (baking foil covered) 6mm and allow it to flex to the correct shape whilst hopefully being able to manipulate the fuel filler neck into the right position through the 5" hole in the former.
I have had a quick practice with 6mm and I think it will work ok. May take a few efforts but...
Will post when I have something.
Thanks for your help to date chaps. :clap: