I am a HR Director.
They should have included it in the original policy, and your argument must be that it’s not good enough to give you quite important information at the very last minute. However, you had (presumably) a whole year to raise your concerns once you did receive it late. It’s only become an issue now that you want to leave.
With that in mind, the choice comes down to bad blood. There’s no way they’re going legally enforce a repayment, it is just not worth the time, money or the “look” of harrassing a new mother, especially when you could easily tell colleagues they didn’t give you the information.
Therefore, I’d settle on a diplomatic / amicable email (which is late in the game but better late than never) that says something like:
Dear Manager,
I appreciate the unfortunate position that the business is in as a result of this key document being missed in the lead-up to my maternity leave. However, I am in as difficult a position as a result, as I planned my maternity leave on the basis that I knew all aspects of the arrangement.
Had this critical information been in the policy, or highlighted to me during any conversations about my leave or return to work plans, I would have behaved very differently in respect of saving money for my leave. I have also spent my maternity leave feeling quite stressed out by it.
I have enjoyed working with the company and don’t wish to leave on anything but amicable terms, but I am not in a position, nor am I willing to be held to a clawback agreement that I did not have knowledge or nor did I agree to.
Best wishes