Neoprene rubber laptop sleeve, sized to take 15.4" widescreen laptops. The same material they make wetsuits out of. Nice and stretchy, will accommodate oddly shaped protrusions from your laptop while giving it a bit of protection.
There were 4 colours in Poundland Cambridge: black, red, blue and pink. These go for about £5 on eBay. Mine was reversible: red on the outside, black on the inside, can be turned inside out. I only found the 15.4" size - no others.
Note picture is of a similar product at another retailer. The Poundland ones have no writing on them.