After staring and fiddling and more staring and fiddling I gave myself a good talking to. If I can't decide what is the best move, then I just need to pick something and then do my best to make it work, so that's what I've done.
In the end I have gone with plan B, which was to use the three smallest lighthouses together in what will probably be the top row. They look like this now.
So far, so good.
I think this project has sparked some of the most interesting, thought-provoking comments I've had since I started to blog (so a huge thank you to everyone who has taken the time to share your views). Several people have suggested that I could do something interesting with the setting for these - for instance using them all laid out horizontally, like a landscape, or finishing them in sets of 3 so that they could be hung in different combinations. Now, I've banged on a lot about my preference for quilts that are big enough to be functional, but in this case this sort of approach seems like it could be very effective and I am drawn to try it out.
There is, though a practical difficulty, which is that these pieces are pretty big (the strip in the picture is about 70" across) and I certainly don't have walls big enough to take them. So this will be a functional quilt, but I am going to start again, making slightly smaller blocks, specifically with the idea of trying out a wall piece.
But let's get this one done first.