Well, having done the painting I decided to see if I could do something about the "plenum" i.e. the bist of flipping plastic wot is broken in two, if I am being polite.
I researched the form of plastic and it is allegedly capable of being stuck together with superglue cyanoacrylate type stuff.
So I had a bash.
Total pain. The long curved multi crossection piece of poop is extremely difficult if not impossible to clamp single-handed. So I put one piece in a vice and ran a bead of glue along its edge, then tried to hold the other piece against it for about 5 minutes. It was only making contact in a few places but ever the optimist.......
Well amazingly it sort of stuck.
But...it is in fact broken in more than 2 pieces and when I tried to insert another little piece, into the gap between the other two, pop, they broke apart and worse still, where I had managed to put a small clamp on the edge to at least hold one part of the join together, it came away and other bits broke off. This being the top edge as you look at it where it touches the windscreen.
Double poop.
So yet again I am playing jigsaw puzzles just to get it back to somewhere handy to what it was before.
I have come to the conclusion that the edges have worn although they look like they should stick together flush.
Things I have learned:
Bits that have freshly broken apart stick back together easier than bits that have been long apart, despite cleaning them etc.
When your fingers are covered in dried superglue to the point where you can no longer feel the tiny little bits properly, a handheld wire brush is quite good at getting it off although it takes P80 paper to finish it!
Enjoy the afternoon better than I am!