What were you considering for your floor? You've gotten this far. Ordinarily wood is what I'd recommend but those pesky nails get in the way of heated floors.

I'd go shopping at a flooring showroom to see options that are compatible with your type of heated floors. Maybe some type of floating floor system.

We know that we can use a floating floor and engineered wood floor. I was more looking for advice re colour or material. We had considered a grey slate/laminate floor, but not sure with the black?

I think a grey could be nice. You already have a lot of wood going on. Also, I believe you can use cork products over heated floors and you can find colors like grey as well. If you go grey, the wall color might need to change to something less golden.