Limestone floor - looks permanently grubby and now has linear cracking - Tile Advice

[Limestone]

Hi, I don't know if anyone can help me. We had a limestone floor laid in our kitchen about 18 months ago. Almost immediately it started to look grubby, but wouldn't clean, I called the tiler straight away and he wasn't really interested, said he'd sealed 3 times as the shop had told him. He said he could clean it and reseal but would charge us. Shop said not their fault, down to fitter. We left it as another store said let it settle down.
The fitter was recommended by a respected tile company (Fired Earth) but we ended up buying stone from someone about 15 miles away, so shop and tiler have no relationship.
When I was getting quotes I was very concerned that the floor wouldn't crack, as it had happened in a rental property we were briefly in. Tiler said he would belt and braces it, so put marine ply down, and ditra matting. Today I have noticed to linear cracks about 2 feet long and 4 feet apart. Wjat should I do? Kitchen has been fitted oover top.
Any help on both issues would be greatfully received, I'm just so disappointed with it.

 

[White Room]

Re: Limestone floor - looks permanently grubby and now has linear crac

Get the tiler back to have a look.....

 

[cam_low]

Re: Limestone floor - looks permanently grubby and now has linear crac

what is the substrate below the ply ? As above get the tiler back.

 

[Concrete guy]

Re: Limestone floor - looks permanently grubby and now has linear crac

Questions:-

1. What is the name of limestone?

2. What is the thickness of the tile?

2. What is the size of each tile?

 

[Turkish tiler]

Re: Limestone floor - looks permanently grubby and now has linear crac

Get the tiler back to have a look.....

As above

 

[Katrina2012]

Now im really worried because im thinking of having Limestone on my floors and in bathroom...

How can one avoid the grubby looking limestone problem?
What thinkness of stone tile is recommended?
What base is recommended?