We are property managers. Our client owns a number of

We are property managers. Our client owns a number of freehold ground rent reversions which comprises a number of leases.
We are in dispute with a lessee over monies due to us.
A sum of £702.39 was incorrectly posted to her account in 9 February 2010.
It transpired that this was incorrect as it should have been the sum of £600.00. It should be noted it was the lessee herself that brought it to our attention the amount was wrong in an email dated 27 May 2016.
Once the matter was investigated we amended our records to show the sum of £600.00 had been paid on 30 August 2011. The discrepancy in dates was as a result of the bookkeeper at the time incorrectly recording a remittance advice for the sum of £702.39 as a direct debit. However, this was noted at the time the £600.00 was sent, though the account should have been adjusted at that time, which did not happen due to staff changes.
In itself the difference between the amounts was relatively insignificant and should not have made any real difference to the account.
However, the lessee changed her view after her email of 27 May 2016 on how the monies should be accounted for by suggesting that as we had agreed an original payment had been made she should be credited with both the sum of £702.39 (as receipted) and £600.00 (shown as paid by her bank).
Though we were happy to credit the sum of £600.00 we were not prepared to credit the sum of £702.39, as it would never qualify as a bank payment, even if originally incorrectly receipted.
We wrote to our own bankers who confirmed we never received the sum of £702.39.
We wrote to the lessee to ask her to contact her own bankers HSBC to obtain confirmation that the sum was paid, if she was so adamant that the monies had been paid. In itself this does not prove we have received the monies but at least helps her to trace the recipient.
Our understanding is that the onerous of proof is on the lessee to show she made payment to us from her own bank, irrespective of whether we had made an original mistake or not.
A further problem has now arisen that as the lessee was in the process of trying to sell her flat she is now implying that as we did not credit her with the £702.39 after requesting it, this resulted in her being unable to proceed with the sale.
There is now an implication that we should reimburse her with the abortive costs of the sale of approximately £3,500 together with other costs.

Lawyer's Assistant: Because laws vary from place to place, can you tell me what state the property is in?

England

Lawyer's Assistant: Has anything been filed or reported?

Yes, this relates to an ongoing County Court Claim

Lawyer's Assistant: Anything else you want the lawyer to know before I connect you?

I hope I have summarised it sufficiently, but have attachments if needed.