Preferably, You'll want to allow for three-5h of transit time in Johannesburg, so that you've got sufficient time for you to capture your connecting flight. Transiting in Johannesburg is very uncomplicated; basically follow the symptoms for ‘International Transit’. The flight from Johannesburg to Harare normally takes about 1h 30m. KAYAK’s https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/