After some more experimentation (I’ve already mentioned that I’d swapped 1g of butter for 1ml of virgin olive oil) I’ve stopped putting any extra fat in the bread at all. On the smaller loaf setting (400g) the top generally breaks a little, but on the 500 or 600g bake the top crust is fairly smooth.
It seems that (in this bread machine at least) the fat just gives a non-craggy top, and so looks more like pre-packed sliced loaves. I prefer the more broken look anyway, even if it can make it slightly more crumbly around the top edge when cut, so feel free to add back in the fat/oil if you want a smoother finish. The basic amounts are:
400g of flour: 15g butter or 15ml virgin olive oil 500g of flour: 25g butter or 25ml virgin olive oil 600g of flour: 25g butter or 25ml virgin olive oil