This is a little something I asked a friend to pick up. Only cost a couple of quid.
You'll probably have to click on it to see it clearly.
Technically, it's a thingy to tighten plastic pipe joints. All it is, is a rather large and thick rubber band in a shaped holder. But it's much more.
It's great for undoing jam jars, sauce bottles and the like. You wrap the band around the top/lid, pull it tight, lean the protrusion (bottom right) against the band, grip it, and turn.
In my case, I hold it between it between my thighs, grip it in my left hand (calm down, madam) and pull quickly to one side. A deceptively simple device, but very useful.
Highly recommended. Oh, and the elastic band in the middle is my adjustment. Stops it slipping out of those grooves.