Read this:
http://www.ukba.homeoffice.gov.uk/sitec ... rnationals
Basically, if you are a South African travelling to the UK before the 30th of June 2009 AND have travelled there before (and obeyed the conditions of your stay), you do not need a visa. However if you've never travelled to the UK OR your planned trip begins on or after the 1st of July 2009, you will need a visa.
Your status in Austria has absolutely no bearing on whether or not you need a UK visa. However if you do need one, you should lodge your application from Austria.
I'm sorry, but I have no idea about your re-entry to Austria. It does seem strange that you do not have a residence permit in your passport. Have the Austrians swapped cards for stickers? This seems to imply that they have:
From wikipedia:
http://de.wikipedia.org/wiki/Aufenthalt ... haltstitel
Alle Aufenthaltstitel werden in Form einer Scheckkarte ausgestellt.
and here:
http://www.wien.gv.at/english/administr ... html#title
All kinds of residence permits and settlement permits have the form of a card and are usually issued for one year.
I would imagine if your little card has the word "Aufenthaltstitel" written somewhere on it, that and your passport is all you need to re-enter Austria. That little card with the word "Aufenthaltstitel" is your visa, as it were.