Fursnewth is Cornish: fos nowyth, 'new wall/bank', with historic forms: Fosnewit 1086, Fosneweth 1196; Fosnewyth 1329; Fosnewyth 1410; Fos nowyth c.1520.  (SWF (all forms): Fos Nowyth)

Lestitha is harder to deal with.  Its historic forms are: Lanstineda, Lansteneda 1201; Lanstenetha 1357, 1428; Lanstynetha 1357, Lestetha 1561.  Without looking deeper,  the site never seems to have been a church site and I suspect that the Lan- part of the name is more likely to be nans, 'valley' (the site stands on a hillspur above two valleys).  This L/N confusion is common in place-names (e,g. Lanteglos = Nant eglos; Newlyn = Lu lyn).  The second element is puzzling:  is its initial S or T?  It would be curious if Old Cornish 'nant' has assibilated and the second element hasn't in the 1201 examples.  At the moment, I can't pin this word down, or even its proper form.  A personal name is a possibility but that's as far as I can go for now.  Suggestions, anyone?  (SWF – on current evidence: Nanstynedha?)

The well is marked on the map but I haven't seen it for years.  Amazingly, none of the books on Cornish holy wells (from Quiller Couch to Meyrick to Straffon) seem to mention it.  Perhaps there's a mention in one of Henderson's works.  The problem is that all too few of these sites have any statutory protection – in 1988, "English" Heritage promised a complete review of Scheduled Ancient Monuments within 5 years.   21 years later, we're still waiting.