I carefully, and slowly, scored the bottom edge of the frame continuously, gently cutting inwards and upwards, until the remaining raised frame was the correct width, more or less. The scratches left on the top section of each 'quarter pane' were then lightly sanded, using worn-out very fine wet n' dry, used wet. It was then polished with a spot of 'T-Cut', although toothpaste will do it too, then dry polished, then given a brushed coat of Future.
Bit of a b@ll ache, and time consuming (especially as I found a suitable spare windscreen after I had corrected the problem!), but it worked out well in the end.