OVO has filed a reply to my complaint with the ombudsman.
They have accepted a meter reading was taken in Oct, '19, and they are proposing to re-issue all SSE bills issued after they took over the retail consumers in Jan. '20, then all bills issued under the OVO name since Jan. 22, thus correcting my meter reading to what it actually is, credit my account with all over-payments, issue an apology and £100 compensation.
On the ombudsman site it's moved from 'Complaint escalated' stage and is now showing it's at 'Prepare Case' stage, two more stages after that - 'Investigate' and 'Decide'.