Hi da,
There is no limit to the money you can have in the bank if you are on Contributory benefit only. Occupational pensions do have an affect on Contributory ESA, the first £85 per week is disregarded and then the ESA is reduced by 50p for every further £1 pension. You should notify DWP about your pension, even if it is low enough that it would not reduce your ESA payments.
Nothing on this board constitutes legal advice - always consult a professional about specific problems