Poor service following a bill query complaint - how do i resolve this?

When I transferred from SSE to Ovo I came across with a credit of around £70.  I usually had a monthly charge of around £150 (give or take) which is about £5 a day; some were cheaper than this. Then, inexplicably, the next month I was charged  £817 - which equates to £27 a day. Then I went back to around £150 a month.  There is no way on earth that I would have used this much in one month. My DD has been put up to rectify this debit. 

I have contacted Ovo via their chat thing but have not had a satisfactory answer or explanation when I have questioned this massive bill.

On 12/06/23 I sent an e-mail complaint querying this bill but other than the obligatory “we acknowledge your e-mail” which they state that they will contact me within 8 working days - I have had no further reply or any form of communication. 

This is a diabolical service from Ovo and I was wondering if anyone else has had this type of poor service? How they rectified it and what I should do to get this bill looked into by someone who is part way competent.

Thanks.