Having visited all the caches in the series (see list below) you will have collected the various letter and number combinations which when plugged into the formula below will give you the location of this mystery bonus cache.
Plug your numbers in here to determine the cache location N 51 0E.(F/D)CB W 000 5G.(A+B)AB
If you follow the route highlighted below from #7 Oliver Cromwell back to the recommended series parking at Steep Church, you will pass the final locations for the three bonus caches and the letterbox (with some small detours obviously).
N.B. The pub toward the end of the route is old, does beer and food, but does shut in the afternoon and only accepts cash.
Caches in the Series to be visited first: