Ladyred: in the last month ... I brought my son a Mac and he spends quite a bit of time streaming and downloading films programmes from his Pc but has always done this
... but not on the Mac. A DVD quality film is about 4GB. It is not complicated to check when and how much data is being transfered. If the ISP doesn't provide this information, you can check it on the computer, perhaps by downloading some software. Your son will have some information as well.
Some Virgin services offer unlimited downloads but they do slow down the connection rate of the top 5% of users during peak times.