So how does the app work? Synced with the housing website Zoopla, it uses your phone’s GPS to track your location and lead you to the door of properties on the market. If you like what you see, you can call the estate agent to arrange a viewing with a single tap on your device. And once you’ve browsed the interior photos, it also shows you floor plans, nearby schools, average house prices for the area and statistics on the neighbours. In short, Knocker is the ultimate in “property porn”.

^{The app from knockerapp.com shows properties for sale based on location}

I’m reliably informed by the app that the South Kensington locals enjoy exercise and travel but don’t do much DIY. They favour broadsheets over tabloids and the average local resident is a company director, professional or self-employed businessman or woman.

I follow the app as it leads me to some forbidding gates in the heart of Kensington, behind which sits a spacious £3.7m flat – with a rather ugly Seventies’ kitchen. A short walk away is a £10m three-bedroom penthouse flat complete with roof terrace gym and Jacuzzi. It sounds irrestibile but the inside proves disappointingly tacky, with glittering chandeliers, crushed velvet carpets, huge silver urns and mirrored walls.

It’s hard to pity the owners of these opulent homes, but using my phone to peek past their impressive facades, it starts to feel a bit creepy. It shouldn’t, I’m assured. “Knocker is a novel idea and a very interesting one,” says Jake Russell, director at London estate agent Russell Simpson. “We’ve all strolled through a particularly beautiful area and imagined living there. This app gives prospective purchasers the immediate opportunity to consider the possibility of purchasing their dream home.” Like many spheres, the property sales market is an increasingly digital world, he says. So Knocker was the obvious next step.

It’s also a gift to the inquisitive, the compulsive browser and the downright nosy. Those who’ve gazed longingly at the brick walls and iron gates that guard Britain’s most magnificent homes can now progress to the next stage of property porn and glimpse the sleek marble and wooden floors within. And the crushed velvet carpets too, of course – that’s the ultimate money shot.