Thanks for the replies. Definitely not too much detail. I have this lifelong obsession with producing the perfect wholemeal loaf. It's an itch that has to be scratched.
The reason for the high hydration is for a lighter texture. When I started baking sourdough, back in April, I was trying to make something like a sourdough ciabatta. I never managed that, but read that higher hydration = lighter texture, and that was particularly important with wholemeal loaves.
I also found that unless I gave the dough a l o o o o n g time to rise, the flavour didn't really come through. I baked some perfect wholemeal boules, great oven spring, but they had no sour flavour and texture was heavy. I also baked one with some very old flour with a high bran content: the surface was pocked with little craters, and I think during baking, the water inside simply blasted its way out through the gluten, leaving behind something that looked like a deflated moon.
Anyway, high hydration plus long bulk rise means that what I put in the oven is an extremely slack dough, certainly too soft to score (the scoring just vanishes during the baking) and probably overproved. I let it rise in a mixing bowl lined with baking paper. Putting in the fridge overnight stiffens the dough, so I can simply pick it up in the baking paper and put it in the casserole. It sizzles and starts to spread immediately. But I will say that the end result is light enough and the flavour is amazing.
I think I will try reducing the hydration and keeping the long, overnight rise. I hope that will mean I keep the light texture and the flavour. This morning I baked a white loaf and a 50% white 50% wholemeal loaf with 75% hydration and they both have a nice shape. I haven't cut them yet, so disappointment could be in store!