Welcome to Flyer Talk jaxthelass

stewardo has some good advice. How long is your connection? On separate tickets even as a seasoned traveller who doesn't check bags I would be uncomfortable with anything less than 2 hour connection at London Heathrow. With checked bags I would want 3 hours or more.

Yes it is a pain having a long connection when travelling with young'uns but it will be more of a pain if you miss the onward flight and your ticket to South Africa (or beyond) is cancelled for not showing up in time.