I think time's a bit tight for a trip to Sandton, bearing in mind the few minutes it takes to get from the Gautrain station to Mandela square.
I would nip across to the Protea airport hotel for an early dinner, and bearing in mind the recent snarl-ups at security and immigration, be back for about 6 to 6.30pm.
You can buy access to the Bidvest lounge for about £12-£15 I think, if you prefer to stay in the airport and the BA lounge is closed. The BA lounge is a bit dull, but the showers are good.