Eacott,
Remember with any clarity, what that change in groundspeed did to your fuel plan when you realized that upon doing your second ground speed check after restoring the decca for the third time in the snow squalls that you were in fact actually getting a roaring 25 kts ground speed on your trip back to Sumburgh? And that itchy feeling somewhere under that set of rubber and canvas jammies , when you realized just how little time you would have to find terra firma based upon what you thought originally a truely extravagant amount of fuel based upon the 60 knots you had planned for! It could be a very long way from the Ninian to Sumburgh some days!