Thanks for the replies.
I think that they have cracked because the tiles are not level (on a slight slope) in the kitchen. Plus there is no adhesive under the tiles.
Sounds like they have been spot-fixed (blobs of adhesive in the corners etc instead of actual being fully covered). If you tap the tiles with a coin you should be able to tell where adhesive is and isn't under them as you get a dull sound where adhesive is suggesting a solid bit and a higher pitched sound, almost a ping, where the
tile had a void under them.
Any
tile that's spot-fixed will be prone to cracking as the weight of everything above it isn't being evenly distributed. In essence the
tile is taking the weight and not the sub-floor.
I'd seriously get that checked by a professional before just doing a repair as I think you may find it'll just keep happening.
And that without taking into account any other problems. You said the floor slopes, that's okay providing the tiles have been fixed properly but if they're now just starting to slope that again suggests a sub-floor issue and the tiles will crack whether they have been fixed properly (with a solid bed) or not.
If you have a sub-floor problem AND spot-fixed tiles the lot will need to come up for sure.