Hi - are you still there?
I was in Fenstanton the other day and had a quick look round the churchyard, but no luck finding Edward and Mary. There are a lot of headstones and most are so worn so as to longer have legible inscriptions. I didn't have that much time, so it doesn't necessarily mean that they are not to be found.
I did find Adelaide Hall, who is commemorated on the headstone of her parents John and Susannah Allin. She died 15 August 1874 aged 42. Also on the headstone are Adelaide's brother Reuben, and sisters Susannah Watts and Sarah Vickery, and 6 (unnamed) grandchildren of John and Susannah Allin.
Fenstanton churchyard is the burial place of the famous landscape gardener Capability Brown - he has no headstone but there is a modern monument.
Huntingdonshire Family History Society do a fiche of monuments in Fenstanton churchyard
http://www.huntsfhs.org.uk/Pubs.htmlIf you search for Fenstanton it is the third item down. £1-50 sounds suspiciously cheap - I wonder if it is a mistake? At GENfair it is much more expensive.
If you were certain Edward and Mary were on a headstone (maybe those fiches give a clue as to where exactly they are - I don't know, I have never seen one) I would be prepared to have another look next time I am in Fenstanton. I took some photos of the churchyard and the Allin grave, which I can send if you are still there.