In reply to Iggy_B:
Are you landing at Jo'burg International? If so your 30 min window to connect to a transfer sounds incredibly tight to reclaim luggage and clear immigration. If you are getting an internal connection you will have to go through the immigration/customs process first. This normally takes me at least an hour. Can't you give yourself more time and book a later internal connection?
However if you did have a bit more time there is a shop at Arrivals (or there was last time I was there) where you can rent a cell phone for your stay. I don't know about Germany, but cell phone calls from SA to the UK are relatively cheap, and a lot cheaper than France or Spain to the UK.