I'd say your first course of action would be to try and trace the Deed of Grant and see if it subsists (ie extended to use by successors in title to the land you own - basically stays with the house and benefits whoever owns it at the time).
I note you said the previous owner said he would provide s stat dec, if you are in touch then could you ask him about the deed itself.
It is possible that it is already registered on your title if the title is registered at the Land Registry. Do you have a copy of the title register you can check, you can download a copy for �3.00 if not.
http://www.landregisteronline.gov.uk/
If not then try and find out what happened to it, check with the previous owners, the solicitors who acted for you on your purchase, see if previous owner can check with his solicitors who acted for him on the Deed or his sale or purchase, basically anywhere deeds could have ended up.
You could try the Land Registry, it would normally have been registered on both pieces of land when produced but mistakes do happen.