Mushroom ketchup is a mushroom flavoured condiment from 'Geo Watkins' - Est 1830 - Meeting House Registered. It is now manufactured by AB World Foods Ltd, Kiriana, Kiribati Way, Leigh, Lancashire, WN7 5RS, United Kingdom. It was purportedly used by Victorian Cooks in pies, puddings, roast meats, soups and sauces.
Its recent popularity is probably due the the Delia Effect: the The Delia effect - the phenomenon which causes her followers rush out to buy ingredients she recommends. She includes it in her recipes for steak and kidney pie and steak and kidney pudding in her famous "Complete Illustrated Cookery Course", and probably many others.
It is commonly available in most UK supermarkets.