Given a travel related situation, and given a specific question about the situation, summarize the question into a title

I am shortly flying from London Heathrow to Windhoek, Namibia with a 5 hour wait in OR Tambo International Airport, Johannesburg. I fly into Terminal A and out of Terminal B at OR Tambo International.

I am flying BA for both legs of the journey, on a single booking.

I have booked time in the Mashonzha Lounge which I believe is airside in the departure area of Terminal A.

Will I be able to reach the Mashonzha Lounge in the departure area of Terminal A, if I am flying out of Terminal B, or do I need to change the lounge booking to a Terminal B lounge?
Airport Lounge in OR Tambo International Airport, Johannesburg