E
Who initiated the review?
If it was the DWP then it is at the Decision Maker's discretion to backdate the award but normal procedure is not to do so.
If you reported a Change then the award should be backdated to the date you did this.
If the former then you can request an MR of the Effective Date of the Decision but you will need compelling evidence that you met the criteria for the higher award on the date that the review started.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems