If Possible, you need to allow for 3-5h of transit time in Johannesburg, so that you've got ample time and energy to capture your connecting flight. Transiting in Johannesburg is rather uncomplicated; just Adhere to the indicators for ‘Intercontinental Transit’. The flight from Johannesburg to Harare requires about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/