They might well be a reputable company, but my concern is that a reputable company would be flooded with work, so why need to involve the likes of Checkatrade at all?
Looking at the quote, I suspect that they might sub out some of the work. The reviews are largely positive, but I have seen work done by people on Checkatrade with a glowing rating and the customer has asked me to repaint the whole room as soon as the guy with a 100% rating had finished.
I have direct experience... a few years ago, she what scowls at me, decided to use one of those websites to get some bloke to repaint a hallway that I had previously painted. He charged her 5 days labour. A week later, she asked me to repaint the hallway. I had to sand the emulsion because of the massive build up in texture. What would have taken me 4 days if I did it from scratch took me 8 days to paint and get it back to the original quality of finish.
I do know tradesmen that get work from those kind of sites but I would never recommend those sites.
The tradesmen above, although skilled seem to adopt a get-in-get-out attitude. I have seen them drop the quality of finish lower than if they were recommended directly by one of the builders that I know.
That said, I am being unfair to the decent tradesmen that use those sites. There but for the grace of God go I, and all that, but I have never needed to sign up to them, equally, I have never advertised. I like the fact that my customers vet potential customers. If their friend is a PITA, my customer will give me a heads up.