Al wrote:
Are those cheap (B&Q, etc) white plastic push-fit end-caps reliable on
15mm copper pipe in a gas-fired domestic central heating system? I
just removed a radiator and used these to cap off the feed and return
pipes. I was alarmed when I saw how easy they push on and pull off by
hand. Has anyone known one to work loose? The pressure of my system
goes up to 2 bar.

1 bar = 1Kg/m^2 of pressure.
2 bar = 2Kg/m^2 of pressure.
15mm pipe has an area a bit more than 1 square centimeter, so you're probably
looking at a force of around 3Kg.
Obtain a 3Kg weight, and compare the forces.