I have been with them for the last 4 years and have just renewed again as they are the cheapest supplier for me for dual fuel.
I once didn't renew in time and was being charged more on the new rate than the one they had said I could renew on. I rang them and said I had meant to renew but had somehow forgotten to renew it in time and they put me onto the cheaper tariff and backdated it for me.
The only other time I needed to contact them was at the end of last year when I had made a mistake entering a reading for the gas and rang them to correct it but the office closed earlier than usual due to the covid pandemic so I had to email them, they replied the following day and corrected it then asked if I needed anything else. As I had built up a large balance, in credit, I asked if they would refund me around £200, which they agreed to do and it was in my account about 5 days later.
I pay by monthly direct debit but I don't think that they send me any notification about when it is coming out, I just get the email asking me to submit a meter reading within the next three days at this time of the month.