Shaun - a couple of things spring to mind here as it's rarely a registration issue that needs addressing as you have to be sure that the deed is both legal and does what is required in law. The registration aspect comes after that so we cannot advise you on whether it is sufficient in law and I very much suspect that is what the buyer's solicitor is getting at.
You are also, it seems, trying to formalise how the pumping station is maintained and access permitted etc to do so. That doesn't read like a boundary agreement and is perhaps something you would really need a
deed of grant/easement which would then include provisions/covenants perhaps re what happens in X or Y circumstances
If you prefer to try and register the deed you have drawn up then you would apply to note same against both titles using form AP1.
However I would strongly recommend discussing the agreement/arrangement with your conveyancer as whether it's 'sufficient' is a legal and not registration issue