The cartridges all seemed to be correctly fitted in place and there was no visible signs of leakage inside the cartridge holder assembly. Had there been a puddle of ink anywhere I would have checked that.
It seems like some sort of "capillary action" with the ink "flowing" through somewhere and not just a leakage.
Many months ago I had noticed a trace of black in the base of the yellow cartridge, which cleared if I printed a couple of "yellow" pages, but now the yellow cartridge looks totally black. Don't know how many "drops" of black ink are needed to turn a yellow cartridge black.
I thought maybe it was a "summer only" problem with the printer being in a south facing room and the ink expanding due to the warmth, but this latest problem has occurred in cold weather.
Most printers have a 'Waste ink collector' usually a pad of absorbent material where overflow from cleaning goes - They fill up and leak onto the platen- this is of all colours mixed - ie 'black' The cure is take the printer apart clean it up - or better still as already mentioned - bin it and get the next one