Does an informal boundary agreement with my neighbour bind future owners of both properties?

Not automatically, and this is a common and costly misunderstanding. An informal boundary agreement, even a genuine, good-faith understanding reached directly between you and your current neighbour, is primarily a personal contract between the two of you. It doesn't, on its own, bind a future buyer of either property, because that future buyer wasn't a party to the agreement and generally had no notice it even existed.

To actually bind future owners, the agreement typically needs to be properly documented, often supported by a reference plan showing the precise area involved, and registered on title, so that anyone searching the title later, including a future buyer's lawyer, can see the agreement and is considered to have notice of it. Without that registration, a future owner on either side can potentially treat the original property description as controlling, regardless of what the previous neighbours privately worked out.

If you've reached an understanding with your current neighbour about a boundary issue that matters to you, don't leave it as an informal arrangement. Have a lawyer formalize and register it properly so it actually survives a future sale on either side.