What are the key ingredients in Worcester Sauce?
DO I has to? Nobody learned it?

Garum, a fermented sauce made with anchovies as the principal ingredient, was the Romans' legacy to Britain when they left GB for home. They put the stuff in everything - Roman Fricassee of Dormouse would be naturally adorned with garum, Roman asparagus would have to be served with garum ...

There was a sauce, Worcester sauce, that had disputed origins - but was obviously based on an English version of garum,. So there was a historic Dickensian precedent a la Jarndyce v Jarndyce between Lea and Perrins, who sold Worcestershire sauce to avoid argument, and Hammonds of Bradford who sold something very similar marked "Yorkshire Relish", which they acquired when they bought Goodall, Backhouse of Leeds.

What happened was that Hammonds won, in that for a brief period they sold WORCESTER sauce without dispute about the name, instead of calling it Yorkshire Relish. Just after that they were taken over by some multinational who didn't give a tinker's cuss about that valuable TM, just the market share. Some brands, including Yorkshire Relish, were sold to an Irish company who are still making the "thick" Yorkshire Relish brown sauce in Ireland, but not the "thin" version we are discussing here.

Lea and Perrins are still selling Worcestershire sauce. The Hammonds Sauceworks brass band are still performing, although the factory produces no Yorkshire Relish. There is a factory somewhere producing Henderson's Relish, which is very similar to Worcester sauce, but made in Sheffield with no Garum - never seen an anchovy.

At least Lea and Perrins still produce the closest thing to a traditional product which goes back 2,000 years.  Whatever they do with the anchovies, that component is said to be left for three years to mature.