I only clicked on this thread because I confused the title with Flat Cats which is a company that supplies screens to put over your upstairs windows so you can keep them open without your cats being able to get out, but I now see that it's like a AirBnB model to connect cat owners with independent cat sitters.
In that case it's likely one of those things where people's experience varies according to the individuals providing the service. I haven't used Cat in a Flat but earlier this year I found a new cat sitter who advertised on https://www.petbacker.com/ as my old cat sitter, who I'd been using for over a decade retired. So that could be another place to look, but Cat in a Flat looks similar.
I googled cat sitter and my town and also looked on Facebook and found a few options, so for each one googled them to find out as much information as possible just to see reviews and if they appeared genuine, honest, trustworthy etc, as it's a big thing to allow a stranger the keys to your home and responsibility of feeding your pets while you're away.
The lady I settled with was listed on Petbacker and had a Facebook page for her business. She had a few good reviews on Petbacker and also Google and her FB page was clearly that of an animal lover with lots of pictures of her, her family, her own pets and also those she boards in her own home - it was the FB page that convinced me she looked OK as it had a few years of posts of her being a pet sitter.
Before committing, she came to meet us and the cats and we had a good chat. We've now used her a few times and it's all been great - she sends Whatsapp updates and photos and most of the cats seem to like her, except the one that doesn't like anyone except me and DP.