I'm a Namibian passport holder, I'm travelling from England via Amsterdam to South Africa. Do I need an airport transit visa for Amsterdam airport?
As far as I understand, no, you do not. According to the Dutch consulate (link to Indian embassy, but it's irrelevant):
List of countries whose citizens are required by all the Schengen states to possess an airport transit visa when they are in the international transit area of airports in the territory of Schengen states:
DEMOCRATIC REPUBLIC OF THE CONGO
List of countries whose citizens are required by the Netherlands to possess an airport transit visa when they are in the international transit area of airports in Dutch territory.
As Namibia is not present in either of these lists, you don't need an airport transit visa.
Namibian citizens can transit in any airport of the Schengen area (including Amsterdam Schiphol) as long as they remain “airside”. You should in particular make sure that you don't need to collect and recheck your luggage as this can create problems (if you booked both flights together with a full service airline and the layover isn't too long this should not be a problem).
In any case, having a UK visa also exempts you from the airport transit visa requirement. So holders of a valid UK visa or travelers returning from the UK after using one (which is apparently your case) never need an airport transit visa, no matter their citizenship.