When we were testing all the more unusual beef cuts twenty years ago, my favourite steak was the skirt. I cooked it rare at a very high heat. The flavour was incredible. But it is often very hard to find the right cut. It all gets very confusing when you get to the underneath part of the cow. There are two different types of skirt: inside and outside. Supermarkets usually sell inside skirt, which is lean, flat and quite thin with a strong grain, and is best sliced fine. But in a restaurant, you will almost certainly have had outside skirt . Morrisons occasionally sell outside skirt as well, but with the same generic 'skirt' label as the inside skirt. That is where I got our outside skirt all those years ago.
The one which looks like this in the Morrisons package is inside skirt. Outside skirt (bottom photo) is quite a lot narrower and thicker with a lot more marbling:
View attachment 408552
View attachment 408554