I wouldn't get too hung up on the policy wording. The issue is that you're making a request. They can refuse it even if you gave proper notice, as it's not what you agreed before leaving for mat leave. I'd say needing staff for a busy Christmas period would be a good reason to want your return from their side, and if they've got a BF policy like you've said, then you don't have a 'good reason' to argue it (good reason in terms of the negotiation of a contract I mean).