Ovo have passed a bill to debt collection agency despite me leaving the property 3 years ago

I received a frightening letter from Ovo stating I had an overdue balance that was transferred to LCS debt collection agency despite me paying my final bill 3 years ago. They said I had been identified through a trace with Experian and it’s for almost 300.

I have been paying my final bill via monthly direct debits but this bill is completely separate to me- not linked to my email or number and for after I vacated the property. I have spent 5 hours back and forth over the last few days and have had extremely frustrating calls with the South Africa customer service who keep telling me to call the debt agency. What concerns me is I am on the verge of getting my first mortgage and my anxiety and the stress of this is sending me over the edge. I have emails from my landlord to state that I had vacated and also emails from my employment at the time as I relocated and was in new work accommodation but originally the property was managed by an estate agent and I had a tenancy but the landlord took it over. I only have emails and not a new tenancy I can prove because I was renting room with an informal agreement when I vacated. I am terrified with the stress and don’t know where to turn now