You adjust the float valve such that it maintains the water level with
enough expansion room between the water level and the overflow to
accomodate the typical expansion.

Ah, yes, of course you do!. Though you can't put the overflow 'miles' over
the (float-valve) fill level to accomdate ~2G (if calcs are correct) of
periodic expansion in 4G tank, periodically submerging the float valve.. So
scaling up to 8-12G in my case looks sensible to me, and might help with the
high temperature blow-out scenaro mentioned. And as I'd prefer to run in
periodic top-up mode (damage limitation if leak) it'll mean less frequent
topping up.

Though I still don't know where to get 8-12G tanks.

Egremont.