Hi Rupert, how did you inform your employer of this, by email or letter? Or just by word of mouth that may not have got to the right person? If I was you I would ensure that you have proof of your reporting of their mistake in writing and then you can point to this, if it ever came to the point that your company requested it back and accused you of just keeping silent. It also depends on what your expectations of your career is with this company. If you are hoping for promotion, I would be very responsible and ensure that you do everything you can to ensure that the money is paid back, earning you brownie points with the company. Regardless, ensure that you protect yourself against any accusations of dishonesty.
Best regards, Sue