Stage 1 - Windmill
Goto N52°36.243 & W002°33.314. To enjoy views of the windmill
Stage 2 - Ammo Box Location
Goto N52' 36.501 W002' 33.825
The cache is an ammo box, containing the usual assortment of goodies.
Look in the log book for details of the BONUS CACHE - GC18HRW, hidden nearby (you will need to return to the car and drive to this location)