The East European, and especially Polish, influence in SW19 continues to grow. You can't get lentils or turmeric any more in my local Pakistani run grocers any more, but you can get matjes, sopoka loin, and something that looks meatish but carries the disturbing English subtitle "headcheese".
Headcheese notwithstanding, the Poles certainly seem to know what they are doing with cured pork and my larder is never without some of their ham, bacon joints or sausages these days.
Matjes turns out to be herring. I bought some because I'd enjoyed it in Glas last month. I served it up with an apple and horseradish sauce and a baked potato because I had read that that was the kind of thing that might be eaten during Lent in Poland.
It may be the result of my imperfect preparation, but it was certainly penance consuming it.
P.S. The estimable Davidson in The Oxford Companion to Food describes herring as being, "of all fish probably the one that has had most influence on the economic and political history of Europe", but doesn't really elaborate on that striking claim in the rest of the entry.