Help please we desperately need heating!!!!!!!!!!!!!!!?

Many combis have built in filling loops. Try the black tap. It sounds very likely. Open it slowly and watch the gauge. It will not be gas as gas is always coded yellow. If nothing happens return it to it's original position. It is possible that it is a water flow or return pipe, but unlikely if there is just one of that type. Don't worry if there is a small water spillage as this is common and quite ok. Have a go, you can't do any harm as taps are there to be twiddled.