Moral, sales people will say whatever to get a sale.
Have you made a formal complaint to BT about the original sale. The discussion about Internet usage is the key here, as if you said email, web and just a little video then most would say 10GB would suffice, but the trouble is what is a little.
e.g. iPlayer HD can eat a Gig or two in just an hour. So 10GB over a month does not last long.
I would also be asked for more detail on the usage billing, as some on in the past have found oddities, e.g. no-one at home and a big bill or usage allocated to days they knew they nothing online.