Hi
I would write back saying that all monies you had received were payable to you and accepted in good faith. I suspect you may have realised that the 2nd cheque you received was not part of your claim, however, if it was clearly addressed to you, you banked it as most people would have.
The company that have written to you should have explained who they were, why they were requesting the money back and under what particular law they have the right to reclaim the money. I would ask for clarification of these details when you write back.
I have dealt with similar cases where money has been paid out in error. In the letters I sent to customers, I was very honest in admitting it was an error, explaining the cause (human error) and that we would not pursue legally due to cost. By asking customers to act with integrity, you would be surprised that most customers sent the money back. They did not wish to spoil there relationship with the company. Only a few customers did not send the money back and we did not offer renewal. The customer was then faced with having to find new Insurers at renewal, having been declined renewal. Obtaining Insurance where renewal has been declined can be difficult as you would have to disclose the details.
I would therefore write asking for clarification but do have in the back of your mind the possibility of a problem at renewal.
Huckster