Does a party wall agreement need to be registered on title to bind a future buyer of either property?

Yes, practically speaking. A party wall agreement that isn't registered on title is much harder to enforce against a future buyer of either property, since that buyer wasn't part of the original agreement and typically has no notice it even exists. Registration is what makes an agreement about a shared wall genuinely durable, rather than just a private understanding between the two owners who happened to sign it.

Once properly registered, a party wall agreement runs with the land in much the same way an easement or restrictive covenant does, meaning its terms, such as repair and maintenance obligations, cost-sharing, notice requirements before alterations, and insurance provisions, continue to bind whoever owns either property going forward, without needing to be re-signed each time a sale happens. An unregistered agreement, by contrast, may still be honoured informally by cooperative neighbours, but there's no guarantee a future owner, who never agreed to its terms and may not even know it exists, will feel bound by it.

If you have an existing party wall agreement that was never registered, or you're creating one now, registering it on title is the step that actually protects its long-term value, rather than leaving it as a private document in a filing cabinet.