The cache is NOT at the posted co ordinates.
Stage 1: Use your OL28 1:25000 map
At the Gibbet Hill trig point, you follow a clear path that leads downhill until it crosses the path of an old leat. Following the leat, you cross the road and continue as it leads to you what appears to be a man-made reservoir. The northern end of the reservoir is fed by a leat so you continue along its path until the source of the leat is located; you find yourself in a steep-sided river valley. Which OS grid square are you now in? Grid = ABCD
Stage 2: Visit the trig point and obtain the four digit number from the base plate. Trig No = EFGH
The cache can be found at:
N 50 (C-A) (F+E) . (A-D) (B+E) (D+E)
W 004 H (C-A+E) . (A-D) B F