I have read online on how to do this and but there is no black unit with space for the white key (which I don't have). There is a black knob with '3 bar' written on it which I assumed was for the pressure. I turned it and it took the pressure to 0!How on earth do I raise it back up?

Emma, October 2015

hi Emma,
you turned the knob to test the pressure relief valve
which will drain the water from the heating circuit. If you don't have the charging key or the black socket to put it in it might not have been fitted (optional).
you need to look for a silver braided hose joining 2 pipes (one 15mm and one 22mm )usually underneath the boiler. there will be a black tap at one or both ends, if you turn the taps inline with the pipe (on position) you should hear water running in to fill system, keep an eye on the pressure gauge and when the black needle reaches 1-1.5bar turn off taps.
reset boiler and should then run ok

Martin, October 2015