Wow, Owain. Here I was wondering if you'd need help installing it and your world looks better than mine!
Really gorgeous. I like it!
I'm a bit confused about lightning starting fires though. Doesn't the rain put it out?
Since I'm assuming it strikes the roof and works its way down, best bet would be to try the stacked half-block trick I mentioned a page or two back.
One thing I've always wanted to try is to build a structure with a layer of water blocks between the roof and the ceiling. Minecraft sprinkler system?