Yep recall the last Northolt Spit well as went past it to school on the A40 everyday in the 70's...my memory says the real Northolt Spit was taken away late 80's and shortly after the plastic one appeared...and I recall it was going to put back into flying condition...
http://website.lineone.net/~landair/wfu.htm
I remember a much later mark Spit at Uxbridge around the same time but dont know that much about it, might have been the same one as my memory is a bit vague