Ok I have now discovered this thread.
http://www.immigrationboards.com/viewto ... ece3d29d74
"To elaborate further, I'm actually waiting at the moment to receive a confirmation of non-acquisition of UK citizenship, which I need BEFORE I attend my citizenship ceremony in order to retain my SA citizenship. Real headache. Once I have this, I'll arrange for my citizenship ceremony and then fire off my UK passport application ASAP. "
It seems from the above quote that I can actually apply for the citizenship before receiving the letter of non acquisition of citizenship. I just must ensure I provide the letter to the South Africans and complete the process before attending the ceremony.
This does not solve the issue of the letter taking 7 weeks, but it means I'm not delaying my citizenship application.
It would be great if someone could confirm.