Number 4 update

OK - I first tried to do this after a meeting in London, while paying my tax bill online at the Traveller's Tavern by Victoria Bus Station.

It was pretty crowded, and I worked out that I'd have to tell one of the bar staff what I was doing, and give them £4 for someone's drink, then give them the nod when someone suitable went to the bar. It was impossible though, partly because the bar was really crowded, and partly because hardly anyone came in on their own.
Next try was in Witney on a Saturday morning when I had a bit of time, at Church Green Cafe, chosen because it's a rare place where you pay after you order.

It only has about four tables downstairs, so the anonymous bit would be tricky. A young couple came in first, and if they'd just had a coffee each they would have been perfect, but they ordered three toasted sandwiches, soup, several sausage rolls, baked potatoes with chilli, and - I think I heard right - a roast swan. Two other young couples also ruled themselves out through sheer greed. A young woman then came in on her own and ordered a coffee. I put myself in her shoes - I would have been the only person who saw her come in, and it would have clearly been me who paid for her coffee. I didn't want to be seen as a pervy old bloke at any point, so I took off her shoes and left.
By Tuesday I'd already opened the next challenge, and had to take action. I remembered there's a chap who goes to the Blue Boar ...

... every day for coffee, who I've met but who won't remember me. He's a good-hearted, gentle kind of bloke (can't name him because people will know him), so I got one in for him even though he wasn't there. So I sort of fulfilled the challenge.