Vokera 20-80 R.S. Flowmatic - Looses pressure and dripping water?

Hi
We have a Vokera 20-80 R.S. Flowmatic combi boiler and for quite a while now (easily over a year) the pressure on the gauge drops to near zero. We regularly put more water in using the two taps below the boiler (we were told to do this by the previous owners rightly or wrongly?!?) and raise it back up to around 1.5. Now, for about the last week or two there has been water on the floor below the boiler. I have looked and it only seems to do it when we run hot water, not when the heating is on. I have just tried running hot water whilst the heater is on at the same time and it didnt do it. When it does do it, it comes from the right hand side (when you are looking up at the boiler). These are pics of the part that is leaking... www.burnleycomputers.com/boiler/one.jpg (bottom of boiler) www.burnleycomputers.com/boiler/two.jpg (leaking part) the water comes from the hole at the bottom. The whole is threaded as if it should have a screw in it like there is on either side.

Any help would be greatly appreciated.

Cheers
Scott