Thanks for the reply.
Assuming that the (undated) web page I've just consulted is up to date, and that there's no difference between economy and premium class flights, South Africa only charges 100 rand as its departure tax for international flights. That's about �7. (Information about the tax on internal flights is sketchy. Several sites quote figures but omit to state which currency they're in! However it seems clear that they're far less than for international flights, so probably only a pound or two).
So, assuming you're travelling before the November increases in UK tax, the total taxes you should pay, for the return trip, ought to have be in the region of �50 if you'd travelled entirely in economy class or �90 for your 'business out/economy back' route. It looks to me as if you may have been charged the extra �40 tax, which your outbounduse of business class incurs, in both directions, rather than in just one.
I suggest querying the additional amount that you've been charged.
Chris