We've got some 800 x 800mm floor tiles in our extension, when the tiler did the job he said there was a slight hump between the old floor and the new, over time some hairline cracks have appeared in some of the tiles, this is not a huge problem but some have got slight chips on the fault lines, is there anything I could use to fill these chips, I did try repair wax but I'm struggling to match the colour (off white with a hint of grey)?

Tiling over any kind of joint between slabs can be problematic. My bet would be there is some movement going on causing the tiles to crack. Wax in the cracks may make it difficult to get anything else to stick now.

 

Not sure what to recommend short of relaying the lot with some sort of isolation underlay. I think even replacing the tiles might only be a temporary solution.