If Possible, you ought to allow for 3-5h of transit time in Johannesburg, so that you've got ample time for you to capture your connecting flight. Transiting in Johannesburg is quite simple; merely Adhere to the signals for ‘Global Transit’. The flight from Johannesburg to Harare will take about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/