No sub meter wightsurf but now SW have made their estimate I'm going too even if i dont use as much as Darren's wife lol! I wonder if anyone who's been on a meter has had a rebate if they used less than the estimated amount - by the sounds of it I doubt it though. 
You do get the rebate bud. 
They credit your account as a matter of course but if you ask them, they will give you the difference. 
Anglian Water for sure. 
The southern companies seem a lot more awkward though...