Hi, More info.
It's not actual mould it's probably more like construction dirt as he has been renovating the house. The tiles are (more like were lol) black and shiny so I may need to be careful with which products I use. So far I've used generic household cleaners and Ubix 400. With the cleaning solutions so far I have used a power drill with a scouring pad and brushes attachments, a glass scraper and a scrub daddy. Last time I went, i used Ubix 400, scrubbed it down and left it for half an hour to work. I then added water and scrubbed it down again then rinsed the product off. When I dried the tiles with a white terry cloth, the cloth came away filthy!! We live in Scotland so it's soft water. I'm at a total loss what to try next. Once I get it clean, I'll apply a coating of wax or Rainex to the tiles to prevent build-up.
My wife swore by these in her own cleaning business for use on tiles, glass, stainless steel, hobs etc. etc. and uses them at home in her retirement too. They're sacrificial so have a short life but is a chemical-free option. My wife and others call them Magic Sponges for a reason.