I currently hold SA and UK Citizenship but I was born in Namibia and am entitled to citizenship there as well. My SA / UK citizenship is all above board with the necessary exemption from Home Affairs. If I apply for Namibian Citizenship will I have to give up my SA or UK Citizenship as both countries allow "Dual Citizenship" - I cannot find evidence of "Multiple Citizenship" anywhere ? Or do I just let Home Affairs know that I am applying for Namibian Citizenship, get the letter and Bob's your aunty ?
It depends on your status in Namibian law.
Disclaimer: this answer is based entirely on Wikipedia; I have no first-hand (or second-hand) knowledge or experience in Namibian or South African nationality law; I have some second-hand knowledge of the state of affairs in the UK.
First, the UK does not regulate multiple nationalities, so you're fine on that front.
Second, on your "entitlement" to Namibian citizenship, are you certain of that? It seems more likely that you already are a Namibian citizen. This is important in both Namibian and South African law. Wikipedia says that those born "in Namibia to a citizen parent or to a parent who is ordinarily and legally resident in the country" are Namibian citizens. If this applies to you, you already are Namibian, you simply don't yet have a Namibian passport or another document showing that Namibia has recognized your Namibian citizenship.
On the other hand, it may be that you aren't actually Namibian, but are entitled to apply for naturalization. This answer will look at both situations.
First, assume you are already Namibian.
Section 26 of the 1990 Act states that no Namibian citizen may also be a citizen of a foreign country. However, this section of the act states that it is subject to the provisions of other laws. In a 2011 High Court case, Judge Dave Smuts ruled that a Namibian citizen by birth or descent could not be deprived of citizenship without consent, effectively legalising multiple citizenship for those citizens. Applicants for naturalisation, however, must still renounce all their other citizenships under Section 5(1)(g) of the Citizenship Act.
Also from Wikipedia:
However, a South African citizen who by a formal and voluntary act acquires the citizenship of another country, automatically loses his or her South African citizenship unless they apply for, and receive permission to retain their South African citizenship before acquiring the citizenship of another country.
Under the assumption that you are already a Namibian citizen, the South African requirement doesn't actually apply to you, and you don't need to get permission to retain your South African nationality. That's because you are not acquiring Namibian citizenship by a "formal and voluntary act"; you already have it.
(Still, it's not hard to see that this line of reasoning might be contested, and you doubtless want to avoid a court case, so it might be a good idea to apply for permission anyway.)
Now, if we assume that you don't already have Namibian citizenship, but are somehow eligible for naturalization, the situation looks rather different. In South Africa, you would need to apply for permission, but that doesn't matter, because Namibia wouldn't naturalize you unless you renounce both your British and South African citizenship.
To reiterate, therefore, the answer depends entirely on whether you are, as you say, "entitled" to Namibian citizenship, or are instead actually already a Namibian citizen.