Ideally, you'll want to let three-5h of transit time in Johannesburg, so that you've enough time to catch your connecting flight. Transiting in Johannesburg is very straightforward; simply Stick to the signals for ‘Worldwide Transit’. The flight from Johannesburg to Harare usually takes about 1h 30m. Critique each company’s pricing https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/