bread question - air pockets

[LeBFG]

So I'm getting good breads: they rise well over an hour and half then good proving (doubling in an hour). I put into 240oC oven which I then turn down to 210oC and get a good spring. All is great except I keep getting large air holes just under the crust!!!! How, why? May this be a poor knocking back or over proving?

I have used this as a chance to look at my new Leiths Baking Bible I quote: "If the dough proves for too long, the bread will have large bubbles under the surface of the crust and may collapse in the oven."

So there you go :)

yay...merci lots wem.