we have an very old bank account which we can no longer use or have access to. There is a small amount of cash in it. I need to write this off in the balance sheet. Question where does the other side of the entry go to in the P/L.
I'd suggest a "Write-offs" account. You could create one if one doesn't exist. If there's a "Bad debts written off" account you could use it. Whilst it's not a trade debt you're writing off it is a debt owed to you by the bank. If your accounting package has a dedicated procedure for writing off trade debts you would not use it in your instance - a bank payment to the write off account would do the job.