Ideally, You should allow for 3-5h of transit time in Johannesburg, so that you have enough time to catch your connecting flight. Transiting in Johannesburg is very effortless; just follow the symptoms for ‘Worldwide Transit’. The flight from Johannesburg to Harare takes about 1h 30m. Whenever you e-book with Kiwi.com, https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/