As it does not seem like we are going to get an explanation from the finder here is mine.
Toe the line at the bank,
This indeed makes reference to the prime meridian, more specifically an art installation called "Time Piece" on the south BANK of the Thames near the O2 Dome.
Reality to the East, me to the West.
Slightly to the East of the "Time Piece" is another art installation called "Slice of Reality" Slightly to the West was the hiding place of the Isis/coin.
Better safe than sorry.
The coin was hidden behind a sign informing you of the number to call in case of an emergency.
The 8.8% inacurracy comes from the fact that when I went back to check that the coin was still there I found that instead of "Time Piece" being on the meridian, 0 degrees, 0.00000 minutes it was actually (according to my GPS) at 0 degrees and 0.0880 minutes.
Pictures of the area can be seen at:
http://www.gold.ac.uk/world/wnews/domewalk.html
Thanks for playing and congrats to whoever found it.
Ade