J
HMRC receives data from all banks in regard to interest paid and they then provide reports to the DWP.
If you do have a bank account with £7000 in it and you are receiving a means tested benefits such as UC, ESA(IR), or IS then you need to report this to the DWP as you have savings and assets in excess of the £6000 limit and there should be deductions from your payments.
if your total assets and savings exceed £16,000 then your entitlement to these means tested benefit ceases.
In both cases, you may have accrued a recoverable overpayment.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
