Allowable Cabin Crew FDP for a 2 sector operation with a post 1800 check in is 12hrs15, I think VS check in 90 mins before so duty starts at 1930 uk time, giving a latest arrival time in JNB of 0845 local which is actually 30 mins before the STA, (the 2nd sector knocks 45 mins off duty) Under normal circs the duty could be extended because crew will have 3hrs in bunks (the min for extension) But this wouldn't be possible with the shorther TUN-JNB flight time. You can add 3 hrs discretion to that, but that still only permits an absolute limit of 2 1/2 delay to arrival in JNB before the flight is totally illegal.
So with say an hour back from the tunrround point and the hour out of tunis to get back where you started it was a complete non starter. Flight crew would probably be less limiting than cabin crew due longer breaks.
I work for another UK based airline and i'm off to JNB tonight, some LH operations are very tight on duty time, had it been a return flight we would probably have got as far as we could and had a crew pax down to meet us but that cant be done out of JNB due lack of crew or suitable diversions