Slipstream etiquette?

Just be pleased that you have helped a fellow cyclist who was in need of some shelter, one day that older, slower less fit rider will be you and wouldn't it be nice to think that you could have a bit of respite on someone elses wheel.

Never bothered me in the slightest. As long as they stay fairly tight, then I'm getting a benefit from them. Wonderful.

Swing out to the right (left on your quaint English roads) and let them through. 

Annoys the hell out of me, too, because it's dangerous - I've actually had a complete stranger crash into the back of me because he was too close when I eased off and he didn't (seems he couldn't take a hint when I'd tried to drop him 3 times).

Sportives, friends and club rides,  when you have a pretty good idea of where you/they are going, then fine, but you wouldn't drive that close to a complete stranger's car, so why do it on a bike?

At least he had the courtesy to thank you.  If it bothers you, flick your elbow after a while and let him have a go.   

I tend to announce my presence, usually by declaring myself to be a parasite, and then take my turn in due course.  There's a certain rudeness involved in catching someone, recovering on their wheel, and then dropping them.  I tend at least to give them a fair chance to sit on if they're minded to.

I have a good working relationship with a chap on an electric bike who shares a decent stretch of my commute.  I sit on his wheel into the headwind, and we have a chat and the lights and repeat.  It all stared when this bloke on a mountain bike eased in front of me at the lights an dthen took off like a scalded cat.  The battery sits in a very discreet, largish under the saddle mountain bike style saddle bag

Say nothing and power away from them...