That's it really. Just been to M&S and he just decided he was in a naughty mood and just wanted to run while we were waiting to pay. Telling him to stay still didn't work. Ignoring him didn't work - he just ran far enough away so I couldn't see him. Even tried distracting him by making jokes about big pants (we were near the ladies underwear dept).
I ended up with him in a kind of headlock between my legs while I tried to hold onto several pairs of school trousers and tried to get my purse out of my bag. He just wriggled and screamed and it just felt as though there must have been a better way of handling it but I don't know what!