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.