The mosaic marble tile is to go into the shower cubby hole for shampoo etc

for the shower niche it's a good choice. they both work well together. I'd use a gray grout in the same tone as the darker part of the marble. Mapei- Warm Gray, or Silver. Custom Grout- DeLorean Gray or maybe Platinum.

The floor tile has a lot of yellow (creamy white) in it.

Is the creamy white something that spoils it?

no. just do the Warm Gray. i have it with my marble tiles and white subways.

It's the perfect neutral. Plus, it's in the niche and the joints are small. you'll barely notice it. Use the warm gray for the floor tile too

So I originally had chosen charcoal hex but the guy at the bathroom place said as it’s a small ensuite it’ll make it feel even smaller, so we have a black marble counter top for our sink and charcoal/black colour radiator to tie in the patterned floor tile, does that make it sound like it will work better?

No, the patterns compete. They're two strong geometrics with no relationship to each other.