Cudeth:
Well of course you can ask for anything you like but perhaps you are asking what chance you have?
First of all I would suggest you drop the interest question but ask for an ex-gratia payment in respect of your "Distress and Inconvenience". The mechanism to do this is to raise a formal complaint strictly in accordance with the EDF code of practice of complaints (possibly you already have done). If you remain disatisfied 8 (or 12?) weeks after first raising the issue you can ask the energy ombudsman to consider your complaint.
You are fully entitled to ask for a refund to your bank account. Make that one of your complaint issues.
The other way of getting your money back is to switch to another supplier. If you have now been confirmed on Online Dual Fuel V3 there is plenty of choice to make quite big savings. EDF Online Dual Fuel V5 is very competitive in most areas though I note than some comparison sites don't offer it to an existing EDF V3 customer. If you want it go direct and ask. I mean insist.
Switching doesn't affect your entitlement to ask for a D&I payment, though it might affect EDF's discretion in making an award. Switching wouldn't affect your entitlement to refer your disatisfaction to the energy ombudsman.
Its up to you. Now that you are wised up to this switching problem you should have nothing to fear from switching and everything to gain.