Do I need a UK visa if I have a 7-hour layover in London? [duplicate]

With a South African passport, coming from the US, this is what you would find:

You’ll need a visa to pass through the UK in transit (unless you’re exempt)

You should apply for a Direct Airside Transit visa if you arrive in the UK on a flight and leave again without passing through immigration control.

Exemptions:

You won’t need a visa if you:

• are travelling to Canada, New Zealand or the USA and have a valid visa for that country
• are travelling from Canada, New Zealand or the USA and it’s less than 6 months since you last entered that country with a valid entry visa

So, when did you last travel to the US? What your US visa valid at the time? If the time is 6 months or less and it was a valid visa, you're fine. Anything else, get a DATV.