Dealing with energy suppliers

I have spent a total of two and a half hours on the phone to Ovo (once I had tracked down a phone number) trying to find out why they think I owe them more than £4000 for energy from June - September for my gas use. As one elderly lady who has not had the heating on it seems like very expensive hot water and occasional use of the hob! I have sent them photographs of today’s meter readings and of my bank statement proving that I paid SSE for my use to the beginning of June, when my account was transferred to Ovo.

Haas anyone else had a problem like this? I am still waiting for a smart meter, booked for early September and cancelled on the day, and am frustrated beyond belief. Doing my blood pressure no good!