Thought I'd mention that I had an email stating that I had exceeded my monthly data usage allowance (provider is Plusnet). This turned out to be a data flow of 7.64 GB in one morning - 100x an average daily amount. My computer had only been switched on for 20 minutes. After 2 hours on the telephone, a Plusnet technical advisor admitted that it was a provider error - helped because other customers had called with a similar problem. One month later I have received a refund of £5 for the incorrectly metered data usage. I was lucky since my allowed additional usage had been set to unlimited - that was not how I left it - now it's set to zero!
Update Nov 2016
On the plus side for Plusnet, the service has been reliable in recent years, but performance took a dip recently. I was down to about 0.5 MBps download speed, with a signal to noise ratio (SNR) of 3dB. After the usual wait for about an hour on the phone, the support person spent over 30 minutes investigating, and after some additional messaging via their website over a couple of weeks, performance is back up to about 2.5MBps / 10dB SNR. So I'm happy with the way Plusnet dealt with it, however the initial fault report has to be made by telephone rather than online, which was a bit annoying.