Thanks guys
Sometimes you get 'snow blind' and miss the obvious things. I winkled it out without taking off the side trims, the top trim came off with the rubber.
I will take the sides off to re fit.
Good call on the rot, early on I wax injected the screen pillars from the bottom, yesterday I spied a handy hole inside at the top, so will get some wax down there and along the front of the roof.
I am grateful to lockdown as the new screen has been with me for 18 months, I have not been looking forward to this at all !
So far all good, the screen channel is unblemished and pristine with factory fresh paint all round.
I really cannot decide whether to fit a new rubber or not..................