SSE Energy Solutions Claimed I Owe £5000 and Left My Dispute Unresolved for Months

I just changed my energy source from SSE Energy Services, and soon after they opened a conflict against my account. This was more than a year ago, and during that period they reassessed my consumption and asserted I owe them an outrageous sum (almost £5,000). Though I have tried to fix things several times, their inquiry has gone on for months without any updates or communication. Since there seems to be no mechanism to raise the matter outside of their own teams, their complaints procedure feels totally useless. Every time I get back to them, I get evasive answers and no actual action. It is quite annoying to lack responsibility and openness, hence I regret ever depending on SSE Energy Services.