Depends if the water inlet is directly connected to the mains. IF it is, it is simply a matter of 'turning on' the inlet valve and presurising to 2-2.5bar.
You should not have a drop in presure over time unless you are bleeding the radiators. If you do, you have a leak somewhere and you need to find it.
***** make sure you cover the from control panel sufficiently with plastic before attempting to fill the boiler. *****
IF it is not directly connected to the mains.
1. You need to take off the cover
1.1. There are 2 screws at the top and two at the bottom of the unit to take out.
1.2. One at the top left (on top of the boiler) and top right as you are directly facing the boiler, and one on the bottom left (underneath) on a 45 degree angle and on the bottom right on a 45 degree angle. Both are phillips heads.
1.3. you then slide off the front cover of the boiler.
2. You will now have to drop down the front control panel.
2.1. Just above the control panel is a phillips head screw in the centre. Remove this screw and the panel will drop forward.
2.2 Cover the control panel with some plastic (so water ingress will not happen as you fill it up.
2.3. you will find on the right had side of the boiler an inlet pipe (black plastic with ridges for gripping)
3. You will now need to connect a garden hose (or similar) to the inlet pipe to fill the boiler.
3.1.Attach the garden hose to the inlet pipe and then push the pipe back towards the boiler and to the left, it should then 'pop' out about an inch.
3.2. have some one else turn on the hose.
3.3. the boiler should now start to pressurise up.
3.4. When the pressure is up to 2-2.5 bar push in the inlet pipe and turn to the right to lock it off again.
3.5 have someone else turn off the hose.
4. The boiler is now pressurised.
4.1 Wipe off any excess water and put back together in reverse order from above.
4.2. You may now have to bleed the radiators for any air taken in whilst filling up.
Hope this helps. I have had to do mine several times as I DIY my new new radiators in the house.
TimLalor
January 2008