If possible, you ought to allow for three-5h of transit time in Johannesburg, so that you've got enough time and energy to catch your connecting flight. Transiting in Johannesburg is rather effortless; basically Stick to the indicators for ‘International Transit’. The flight from Johannesburg to Harare can take about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/