First Direct can't find £550 cash I paid in and credited my £1,200 in the wrong account - but is now threatening me with court action

I have a First Direct account. I paid in £550 cash at the Aberystwyth branch of HSBC to pay my credit card account, as I do every month. When my Visa statement arrived, the amount had not been credited, so I asked the branch to trace the money.
Soon afterwards, I went to the same branch and gave the same cashier £1,200 to pay an invoice. He credited it to my Visa account by mistake.
Visa would not refund the money to my HSBC account, so HSBC head office agreed to put £1,200 in my First Direct account so that the invoice could be paid.
I have spent 14 months trying to get them to trace my lost £550, but they just demand the £1,200. I have paid £650 (i.e., the £1,200 minus the £550).
I have recently received a court order. I am feeling desperate, as they ignore my letters and have made no effort to find the lost money. L.A., Llangeitho.

In more than two years on this column, this is the first complaint I have had about First Direct. That’s a measure of its normally excellent customer service. This time something has clearly gone very wrong.
The error seems to stem from the HSBC branch. Your full letter vividly describes the difficulties you faced and in your place I might use another branch. First Direct’s response once I intervened was terrific.

Branch problems: Always check your receipt when you pay cash in over the counter

They still cannot find any record of your £550 deposit and I doubt they will now.
But in view of your excellent record and after reviewing both the personal and business circumstances surrounding your case they are going to settle the outstanding debt of £812.13 as a goodwill gesture. This includes £262.13 legal fees.
Your case is a lesson to readers that if you pay cash in over the counter you should check and file the receipt carefully so you have evidence if something goes wrong.