Question Author
I have since found out that the dwp gets information sent to them from the inland revenue on all bank accounts that match people on the dwp's list of persons who are claiming benefits, i.e. income support, job seekers allownce or any other benefits. As to whether or not they are allowed to access your accounts without your permission I still don't know, they didn't get my sons permission to access his.
Remember if you are claiming benefits you should inform them of any changes in your income status because the paper work will get you in the end.