Turns out,that this odd tower,was an early Radar Tower.Typically British then,that something so important,in it's time,is being allowed to rot away before our eyes.
see http://en.wikipedia.org/wiki/Coalhouse_Fort
NB:If you do read the wikipedia notes,it talks of "Two Forts Way"a public footpath.IT FLOODS AT HIGH TIDE.AT HIGH TIDE THE PATH IS UNDER 4 FEET OF SEAWATER ALONG THE FRONT OF THE POWER STATION.