By trial and error I discovered how to do it:
(NOTE - Be aware, the Key is a "stopper" to stop water from flooding out of the system when you charge it; the Key is not the actual tap)
1) Insert the key into the hole in the base of the boiler by lining up the arrows on the key with the "unlocked" padlock symbol (use a torch and you'll see the padlock symbols). You have to push it quite far up. Turn the Key to the "locked" position. (about 1/4 turn from the position you inserted it in)
2) You'll see a square plastic knob sitting beside where you stuck the key in. This square knob is the actual tap. Turn it slowly (I managed to use just my fingers to do this) and you'll hear the water charging in and see the pressure gauge start to rise.
3) When the pressure is correct (around 1.5 bars) turn the square knob off. Then turn the Key back to the "unlocked" position and remove it. You'll get a few drops of water come out with it but that's normal.
NOTE: Whatever you do, DO NOT turn the square knob without the Key inserted and in the "locked" position. If you try this (like I did) you will get a shed load of water come shooting out of that keyhole!!!!
Isn't it nice that they don't put any of this in the instructions!
Hope this helps.
Gordon C.
March 2008