This company is trying to exploit a loophole that they know are borderline illegal, and if I take them to court I would definitely win, I just don't have the time to. (And it's only a matter of time before someone else does)
Collating from my own experience and other reviews, they dish out meaningless price increases and surcharges, makes no effort to communicate the electric bills (which violate OFGEM rules), and then hit tenants of apartment buildings with a large bill at an awkward time. They also offer no alternatives other than themselves, which means no valid consent was given to them to act as the "billing agent".
I myself had been billed with >£1k in bill AFTER moving out, without any warning. After calling (and calls not going through) and then being ignored and poorly handled by many different agents, I have finally decided that their business practise was unethical, wasting my time, threatening my health and wellbeing by wasting my energy.
On the bright side, they didn't even bother to keep track of my previous communications and whether or not I have logged into my account to view the bills, so they actually can't say that they've given me sufficient notice, or have billed me correctly, because they haven't given me any evidence that my meter reading was the "actual meter reading" taken by their staff.
Everytime they send me a "reminder" email to pay my bills, I just ignore them, or send them an angry email in reply. In the end, they can't send me to debt collections (they don't have my address), nor does it affect my credit score (they are terrible at keeping records, and even if they have the record, I'm happy to take them to court/ tell my bank that they're scamming and that I didn't sign a contract to pay them, so I don't owe them anything).
And just remember: you only need to ignore then for a year and you're scot free: (from Ofgem website)"You can't be charged for gas or electricity used more than 12 months ago if you have not been correctly billed for it, or informed about it via a statement of account, before."
In conclusion: Ask to see a copy of the contract & terms of conditions with YOUR signature on it, and see how they shriek and tremble in terror upon realizing that they have no legal grounds to charge you.