The 12 week retention excuse is an industry wide thing but I would imagine that 12 week storage problems is actually rubbish as an excuse for not doing it. For audit purposes information must be legally stored for 6 years. I would think that custmoer accounts with balances that form part of
William Hills balance sheet at any point in time are an important part of this.
The real reason is I suspect is that the bookies fear that staff would be "swamped" with requests for old balances.
Like Merlin I think that what they are doing is highy suspect and would be found wanting in a court of law.