In order to find this cache you'll need to visit several others* - or five AdventureLab waypoints (see waypoints below) - to gather information from them:
Caches in the 'Simply Scilly' series:
Scilly String (St. Mary's, IOS) - this Final or Bonus cache, tying them together.
Scilly Walks (Bryher, IOS) - A nice walk on Bryher.
Scilly Sausage (Burnt Island, St. Agnes, IOS) - Clue is in the name. Note: TIDAL!
Scilly Putty (view of Gugh, St Agnes, IOS) - Originally near a golf course..!
Scilly Haircut (Samson>St.Marys, IOS) - Not on Samson, but a visit is recommended.
Scilly Season (St. Martins, IOS) - A Wherigo cache; download the Cartridge in advance.
Scilly Billy (Tresco, IOS) - Set in 2012 as IoS first Letterbox/Hybrid. Find it in two ways.
Scilly Questions (Somewhere on IOS) - A nice easy puzzle.
Take the data you've gathered and apply them to the following formula:
N49° 5A.BCD W006° 1E.FGH
Where a=(St) Agnes, b=Bryher, t=Tresco, m=St Martin's and s=Samson/St Mary's, and:
A=((m-t)+a). B=(a+t). C=((b+t)-s). D=(b+m+m+s).
E=(a+b). F=(m-s). G=(a-b). H=((a+m)-(b+t+s)).
*As getting to Samson may not be so easy, it is possible to cheat and find this cache without finding the St Samson cache by... guessing a number, running the formula, mapping a waypoint, and considering its likelihood. The cache is NOT hidden in a dangerous, wet (e.g. in the sea!), or private location.
RIP Harry's Wall. 2001-2015. A fine ancient cache on St. Mary's. Gone but not forgotten.
Read more about The Historic Geocaches' campaign.