Transit at Heathrow for Schengen to SA flight - using US visa in expired passport. Experiences?

Hey guys. Been rerouted on BA and will transfer between Terminals 3 and 5 for our flight from a Schengen country to Cale Town. The UK website says that you don’t need a UK transit visa if you have a valid US visa - which we have - but it’s in expired passport, which is fine for travelling to the US itself. Has anyone tried this for the UK transit?