It's an Epson 830 (something like that).
It's just started leaving dirty irregular (black ink) )patterns down the page (on the print side).
I had a go at cleaning the little paper feed top rollers and it kept on happening. Further careful investigation shows that after cleaning the ink from these rollers, they are wet again after the print head has gone past. There is also quite a build-up of wet black ink on the plastic bits that hold the rollers, but this doesn't get onto the paper.
Can the print head be leaking or overfilled? and what can I do about it?
I got a temporary fix by printing in dark blue instead of black, after three dirty sheets I got a clean print. So it seems that the black is only overfilling or leaking when black is actually being used.