interests like these are not called "progressive interest", they are just called "late payment interest" but according to you, if you are paying late payment interest for the redemption sum, that means you are already in your Extended Completion Date. You will continue paying interest until the settlement of the Balance Purchase Price which should be maybe close to 2 months from now.

i think you better ask your solicitor to explain to you why are you paying late payment interest and whose fault is it that the disbursement of the loan is delayed?