To get a charge placed on your property, your ex would have to:
1. Send you a formal, written demand, with a specified time limit for payment.
2. Wait until you'd not paid by the relevant date.
3. Commence court action against you.
4. Convince a County Court judge that you owe the money.
From what you've written, that seems to be very unlikely but, assuming that judgement was given against, you'd either be ordered to repay the money straight away or (possibly more likely) in affordable instalments. Your ex then couldn't proceed further unless you failed to make the repayments. Only then could she seek enforcement action (including getting a charge put on the property).
Simply tell her that you've no intention of paying and leave her to decide whether it's worth initiating court action (which she'd probably not win).