Like a cork in a bottle. The U-bend arrangement was not the proper shape and the horizontal pipe leading to the drain did not angle down and away properly and so the gunk had no reason to go anywhere and so it set up home in the bottom of the U and solidified.was it blocked after all?
Was very nasty work to cut out the current pipes, scrape out the horizontal pipe and replace with a decent trap. Now, should all be hunky-dory.