There it was, sitting on a table at the Flea this afternoon: a lovely piece of Imari style ironstone that I took to be about 1870-ish. With a Japanese vase figuring large in the pattern, I found through a few minutes googling that- indeed - the pattern is called 'Japanese Vase' and was made in England around 1875 by Ashworth - a company that bought Mason's in the 1860's.
It's a meat dish or serving platter, about 12 inches long and has been repaired with 4 staples - a peculiarity that only adds to the piece's patina. Someone valued it very much - or a least was thrifty enough - to keep using it. Perfect for scones at tea time.