I and my partner moved out of a rented flat in March this

I and my partner moved out of a rented flat in March this year after staying there for about 18 months. After various back and forth over deductions from the deposit, the management company repaid the bulk of it and we continued to argue about the final few hundred. Finally I gave into most of it just to get it done though I thought the deductions were unreasonable (they charged 300 for cleaning for example). They agreed to repay around 77 pounds and sent us a final agreement docusign pdf document to sign and return. We did this. They then went quiet. I chased and chased. Finally I heard back saying they'd unearthed 600 worth of heating bills that the landlady had paid by mistake over the last year. They said they'd invoiced us over the last year and asked us to check if we'd paid. They said they'd only just been informed (by the external supplier) in spite of having previously checked and confirmed final figures and been told we were all paid up to date. The situation is confusing as sometimes heating bills came from the management company and sometimes from an external supplier who provide the hot water and communal energy to the block (newish build block of flats.) As far as I know we had paid all invoices received but it's possible that something was missed due to the confusing manner of invoices, no-one telling us we hadn't paid and then being told we were up to date etc. Most recently we had a final bill for 43, one before that for 67 which I know I paid direct to the energy company. My questions is, does the management company have the right to demand these payments of 600 now that the final agreement is signed and they'd agreed to pay back 77?
Also, we actually had major hot water problems for a period of about 5 months where we only had a literal trickle of hot water and struggled to get them to get it fixed. Eventually it was done just before we moved out.
JA: Because laws vary from place to place, can you tell me where the property is located?
Customer: london uk
JA: Has anything been filed or reported?
Customer: no
JA: Is there anything else the Lawyer should know before I connect you? Rest assured that they'll be able to help you.
Customer: not that I can think of