Looking at the capacity graphs (http://www.plus.net/support/service/network_performance/broadband_bandwidth_usage.shtml) it looks like the peak time for plusnet is around 8pm to 10pm. It's probably a similar story for most other consumer ISPs.
So am I right in understanding that the ISP pays for sufficient bandwidth for that peak; all usage at other times is effectively 'free' for the ISP? I.e. their charges are based on the peak usage rather than averages or similar?
The reason I ask is that I'm aware I'm a heavier user - despite being on the Unlimited package I'm making sure that my bulk usage happens between midnight and 8am (the 'off peak' hours for Plusnet's usage based packages). Is it worth me continuing to do this?