I like rollmops for lunch occasionally, but they haven't been available to order online @ Tesco for ages. However last evening I spotted some in our local farm shop (they'll probably be gone in a few weeks, they trial a lot of stuff and never re-stock it, like the best naans I'd ever bought in a shop) so decided to buy some. And next to them I saw, from the same manufacturer, sweet cured herring. I checked the ingredients, and noted that the sweet cure had sugar (no surprise) and acetic acid, whereas the rollmop have white vinegar. But what took my eye was the percentage of herring. 39% in the rollmops, 50% in the sweetcure. Same size pot, same price. Sweetcure it is then.
ETA if they're any good I might stock up, given the use by or whatever date was September time.