http://hexus.net/mobile/news/service...ng-complaints/

Vodafone has been fined a significant amount following investigations by telecoms industry watchdog Ofcom. In total Vodafone will have to cough up ?4,625,000 as a penalty, after two Ofcom investigations uncovered "serious and sustained breaches of consumer protection rules".
The first investigation looked at Vodafone's PAYG operation. It was discovered that over 10,000 PAYG customers didn't receive the full top up credits that they had purchased. Over a 17-month monitoring period Ofcom says that the affected customers collectively lost ?150,000. That's equivalent 15,000 x ?10 top ups disappearing into the ether.

Apparently the disappearing top ups resulted from Vodafone transferring to a new billing system. However, Vodafone was judged to have "failed to act quickly enough to identify or address these problems," as Ofcom had to intervene to prompt Vodafone to reimburse the affected customers.