Her old flat was in Tooting, not Clapham. They bought it for £238k in 2011 and sold it in 2016 for £500k, having gone up into the loft and converted (and also taken advantage of the hike in propery prices then). So they made £262k profit on that place, less whatever they spent on doing it up.
They bought their current place for £630k. So, not including any money they’ve paid off the mortgages or adding any for the works they did to either place, they would have had to have a mortgage in the region of £345k. Which means they have either had family help or she’s telling porkies about their income.
And Alex, you might be worried that I know all this, but it was easy enough to find from the information you posted on a public forum (rough locations, moving dates, pictures of both places).