In which year is this taxable

Somone claimed ESA.  It was paid for a short period then stopped by DWP.  They appealed against the decision and took it to tribunal.

Person won and it was paid in 2011/12.  Arrears from September 2011 were paid this year also.

When are the arrears taxable?  Is it in 2010/11 or 2011/12.  Logic suggests it should be taxable last year as that was when the entitlement arose?

Can anyone help?