Usage meter
PC or MAC, doesn't the router have a stats page which shows total download since last reset ? Over a few days you could at least get the average and then extrapolate away. Depending on how clever the router is it may also offer parental control which lets you switch off every hour or so by way of disincentivising.
Also, does sky count all usage or only peak time ?
The things to watch are music and video downloads, especially torrents.