Say, "Excuse me, sir... I think you're riding a bike that was stolen from me..... sir...... sir..... HEY! GODDAMMIT!" while he looks at you and eventually rides away.
I'm going to guess you live in the UK, seeing as Sainsbury is a popular supermarket chain there..... and assuming as much, I'm going to assume you can't do something like buy a shotgun without having to go through a lot of hassles involving licensing.
I mean, I was going to suggest blowing the fùcker away while he's still on it.......
Actually, if someone stole your bike by cutting your cable straight off, they were probably in serious need of the proceeds from selling the bike. In fact, if they're stealing your bike to sell it, they're probably doing it fro drug money. I suggest not confronting people like this, even if they did steal your bike, unless you wanna get stabbed. Just get another bike, and next time, mark it in such a way where you'll always know it's yours..... in such a way where the markings are indistinguishably recognizable, and easy to spot from far away. Seriously, no one wants an ugly bike they can't sell.
If someone stole your bike, they probably did it for drug money... and they're probably not the most reasonable of people..... I mean, they can't be if they stole your bike from such a public place.