Vw polo 1.4 alternator problem? or not?

My 2004 vw polo, aternator symbol light came on while driving and then the ecu light,not long after all the electrics failed and would not turn over to start..
I charged the battery and measured the voltage as the the car started ok, but keeping an eye on the meter i saw the voltage dropping gradually, is it the alternator?..somebody help..and are alternators difficult to replace, I also noticed a tiny flashing spanner light over on the rh side of the dash....grateful for any help