First Utility failure to issue bill after a year

Hello all, I'm hoping you can offer me some advice on how to deal with an issue I've had with First Utility for nearly a year now. Apologies in advance, its a long one! There's a TL;DR a few paragraphs down...

Our tenancy started on 1st June last year, when we moved into the property on the 18th, I rang up First Utility to create an account and to get the electricity switched on (prepaid meter with negative credit). While on the phone I arranged for the meter to be switched to a credit meter and for the gas to be transferred onto the same account. A few days later I completed the credit check and all seemed good.

By October I had still not received a bill so I phoned up the customer service line, I was told it would be sorted and an 'internal communications error' had led to the problem.

By December we still had not received a bill so I sent my first complaint by recorded post (automatically acknowledged by email) stating my concern and requesting we be back billed for the appropriate amount on the most favourable tariff (we had not had any communication as to what tariff we were supposed to be on). Later on in December the automated texts and emails started rolling in demanding payment for the account and threatening my credit rating. We had still not received a bill!

In January I sent another complaint by recorded post (again acknowledged) and requested again that a bill be issued on the most favourable tariff and we be compensated for the trouble. I also asked that the threatening messages be stopped and when a bill did come through, we should be able to pay it in installments. Two weeks later I received a call stating that the account had been frozen while this was being looked into. We received a wildly inaccurate bill in the post with no context whatsoever (I had provided meter readings in every letter) and were advised to ignore it and a new one would be issued.

By February, still nothing! (see a pattern here?) I complained by email which was acknowledged and requested again that a bill be issued, the threatening messages stopped, payment by installments and compensation. A week later, I had an email stating that a bill would now be issued. In mid Feb I complained again (number 4) to the customer service representative's manager to which I received the response that it has been 'prioritized'.

By the beginning of April I was completely fed up, I complained a fifth time directly to Phil Gripton (the Chief Operating Officer). I also stated that I would liked to achieve a resolution within 6 weeks as we would be moving away from the property (our tenancy ends on the 31 May). 5 days later I received a response from yet another customer service agent stating they were looking into it but then nothing. At the end of April we received another inaccurate bill in the post requesting payment in a 2 week timeframe but for gas only. I sent another message stating that the bill was inaccurate, reminded them of my expected outcomes (correct bill, stop to the threatening debt messages, payment plan and heavy compensation) and to cease the contract (which we have never signed or been emailed) on the 31st May.

On 10th May I received an email requesting a gas reading and asking me to confirm that we have a pre-pay meter (incorrect), gas meter was exchanged to a credit meter (also incorrect!). I responded immediately with gas and electricity readings for the entire tenancy and corrected their assumptions.

Since then, nothing!

So what to do? Its been 7 months since I raised the initial complaint and 9 months since I made them aware of the issue.

When we move out on the 31st, can I refuse to pay? I believe I have the right to compensation to be knocked off whatever bill they concoct. Complain to the ombudsman? (but what will they do?)

We have just over a week left in the flat and we will certainly not be using them in the new place! Who would have thought it would be so hard to pay money to a utility company!