Main description in Hatterrall Hill 1
A very easy multi, just for a bit of variety. At the co-ords you'll find a boundary stone with a name on convert those letters to numbers using a standard telephone key pad to give you six numbers which will represent ABCDEF. So co-ords for the cache are
N51 55.(F-C)B(A+E) W002 59.D(A+C)(B/F)
Then continue on the Offa's Dyke path to the next.
Many thanks to the Brecon Beacons National Park Authority for giving permission for these caches.