If Possible, it is best to allow three-5h of transit time in Johannesburg, so that you've got sufficient time to capture your connecting flight. Transiting in Johannesburg is quite easy; simply just follow the signs for ‘Worldwide Transit’. The flight from Johannesburg to Harare will take about 1h 30m. KAYAK’s https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/