Part 2 - Pay on the Nail
If you have found the Part 1 cache, you will have discovered that is hidden next to a coal post. There is also a clue in the cache page - "not too taxed".
You now have to find out how many of these there are round London. The number I found works with the decoding scheme you worked out for Part 1, the decode is to be found nearby, and the number you want also appears on the cache page.
The number of Opens can be found on the plaque I sent you to in Part 1, or you can Google Walton Heath golf course and find out how many European Opens were held there - answers happen to be the same.