Civil Penalty for failure to notify

I have a case where a client has a civil penalty of £50 in respect of an overpayment of income support.  He states that he informed DWP that he had started work.  He did this by letter but has no proof of this, and Income support continued to be paid into his post office card account (which can only be used for DWP payments).
About 2 months later he received a letter from DWP about his income support and he informed them for a second time that he had started work 2 months ago.  Again they failed to stop paying him.
When they discovered that they were still paying him, they asked for the overpayment back.  They accept that the second half of the overpayment is not recoverable, as they admit official error (they did not act on his second letter which they admit receiving).  However, they have asked for the first half back, as they deny ever receiving the first notification that he started work. The exact amount of the overpayment was sitting in his post office account and he immediately repaid it (and the part resulting from official error has since been refunded to him).  He had no idea that it was there because he had no cause to look at his post office account as it was only used for his DWP payments, which he believed had stopped.

He has a tribunal soon in respect of the £50 civil penalty, which he strongly believes that he should not be required to pay.

Any thoughts?
Thanks

Thanks very much for this.  My client is not really interested in appealing against the recovery of the overpayment - he feels that it was never his money and so he should repay it (!).  It’s just the civil penalty that he objects to.  I agree, that he only ever acted in good faith, and given that the DWP deny that they received his first letter, and then proceed to fail to act on his second letter, I think that credibility lies with my client?  He certainly has credible reasons…