Hi sorry for not replying, my notifications had turned off for some reason.
So, to answer a few points:
Yes the boat is metal, but the doors and the frames at the front are wood, so I would need them replacing in wood.
Yes, I know it will make it a bit darker, but only a little as it has windows either side, and the door itself has thin windows - as you can see on the photo, the curtain covers half the glass anyway.
It also has a cover, so it doesn't get wet as such, but I'm hoping that it won't leak.
I had some advice from a carpenter who said: just take the glass out and cut the shape of the wood from that, then fit and fix the piece and re-bead it. Easy said than done for me, but that sounds logical.