To close off this thread, I bought a couple of TD5 rear doors and stripped off the window frames. At first glance they look the same as a D1 however they are different enough (in their fixings) to make this a difficult modification. I'm now back to trying to patch up mine.
What did strike me though is that these door tops (window frames) suffer from the iceberg effect in that when you think there's just a little bit of rust (as was visible on these TD5 ones) you pull them out of the door and find a whole load more!