Surely it must be possible to dismantle the frame and take it to a glazier to have him put the requisite glass in for under £100? The only problem I foresee is that they won't be able to obtain glass with the heated elements but if it will save £500 or more, it is well worth considering.
As a matter of interest, Caterham sell their own similar but slightly smaller, heated screen assembly incl. alloy frame for under £500.
I wonder how many bits of profit are built into the Morgan price? But there again it is what has kept them going for over 100 years and given them a healthy trading profit last year.
