Jennifer
I'm afraid you are going to lose the extra £15.90, this is paid as Income Related ESA so the pension will be deducted £ for £ from it, there will be no further deduction from the ESA(CB) part of your payment. This will be from when your pension started so there may be an overpayment as well.
Unfortunately this also means that you will lose entitlement to the passported benefits that ESA(IR) gives you, free prescriptions, dental care, etc.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems