I've decided to do it the "Volvo" way - as suggested on this forum. I'm not sure if it'll work.
As far as I can understand the Volvo way is the proper way in which it is meant to be done - but - this way is the way that might damage the trim (even more than it is already). The "Mercedes way" is to fit the trim to the seal before fitting the window seal and trim in one go to the car.
So anyway this is attempt #1
^^^Here I've fitted the seal to the glass and then fitted a cord around the outside of the seal^^^
Went in nice and easy =>
With the window in place the next step is to fit the trim - this is meant to form a rigid fix
Quick polish of the trim =>
Now I've got to find the missing windscreen fitting tool...