Well, thanks to the nice folk at A8 Parts, I now have a double-glazed 2.8! It's lovely and quiet. We had a couple of hic-cups because mine has blue tinted glass (most are brown, apparently) so Dave had to scour his stock for the right coloured glass and was very careful to make sure all windows were unscratched and clear (you can get moisture beween the panes, he says, that makes them slightly opaque).
But I'm very pleased with the result (having tested it perhaps just a little over the speed limit and didn't even have to turn the radio up!) and, yet again, I'm astonished at how helpful, courteous and professional the guys at A8 Parts are.
THANKS DAVE AND TEAM!