Right, well I got the book name wrong (it's Old Stones of the Cotswolds & Forest of Dean), but it does have an entry for the churchyard stones at Elkstone, and the entry includes reference to a possible destroyed long barrow:

The Archaeological Record records the possible existence of a long barrow site north west of High Cross as SO964135. All that remains is an elongated oval scatter of limestone in the cultivated field. A standing stone once stood in the field which may have been a stone from the barrow. This was removed and is believed to be the stone now set by the wall. It was once used as a marker for the Coberley Parish boundary (G.T. Harding, July 1961).

However, "Gloucestershire Barrows, Lists: Long Barrows, Gloucestershire", H O'Neill and L.V. Grinsell 1960 (Transactions of the Bristol and Gloucestershire Archaeological Society) doesn't mention any long barrow at Elkstone.

I've also checked the 19th century 25" maps and there's no long barrow shown in the vicinity of Elkstone. Interestingly the round barrow in Emma's picture wasn't shown as an archaeological feature (marked "tumulus") into the 20th century. It was shown as a clump of trees prior to that!