A quincunx is an arrangement of five dots as seen on the
fifth side of a playing die, with one at each corner of a square,
and one in the centre. It is named after the Roman coin worth five
twelfths (quinque + uncia) of an as, which was
sometimes emblazoned with that pattern.
The cache is not at the above co-ordinates which is just the
mid-point of the other four caches in the series (thus forming a
kind of distorted quincunx); it's a fair distance away, and there
is nothing to be gained by visiting the given co-ordinates. To find
the true co-ordinates, you will have to collect at least three of
the four numbers, labelled A to D in the other caches
in the series:
First Turn,
Second Hole,
Banbury Road and
Water Eaton. Since these numbers add up to 10,000,000,000 (the
tenth root of a googol), you only need to find three to be able to
work out value of the fourth. A – B + C –
D, with the prefix "SP", gives the Ordnance Survey grid
reference for the cache. I can't think that any particular
combination or order of the first four caches will make the bonus
cache any easier than any other order. Whichever of the four you
end on, this cache is still at least two miles away. The cache site
is best approached by bicycle, in my opinion, although it can
equally well be reached on foot.