Put the key in.
Flip it over.
Take off the metal plate.
Take off the locking bar.
Look down the gap between the barrel and the plastic.
Use a small screwdriver to flick the retaining plate into the barrel.
Twist the lock the wrong way to spin it out.
With some barrels if you take the key out when the barrel is out of the unit the pins fly out. Others self retain. So to be really sure wrap it in cardboard and sellotape to hold them in place to release the key.