Main description in Hatterrall Hill 1
An easy multi along the same lines as the last. 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 nine numbers which will represent ABCDEFGHI. So co-ords for the cache are
N51 54.(A+C)H(D-E) W002 59.GC(B+C)
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.