Goosey, you've mentioned this before, and I don't think a GPS would be accurate enough.
I'm afraid you need to get a few young guys and gals up there with a tape measure, some tinctures, some posts, a few more tinctures,and some permanent markers to show the extent of the old buildings.
If I was closer, I'd do the job, but Kent to Cockersand is a bridge too far, sorry to say...
Why not ask a local surveying firm if they want to give field experience for their surveyors? They have to do a practical project for their finals, and this would be a superb way for them to do a scheme!