Graham I am worried.When we got our YBS he had a kinked tail which didnt bother us we just thought it had been trapped or bit by another turtle or something.We have had him 2 and a half week and it has gradually come off ! He now only has 3/4 of a tail what shall i do if anything?

Sounds as though it may have been in a scrap! Minor injuries like this generally heal quite quickly by themselves, as long as there is access to a basking lamp and a UVB source to promote healing, and obviously the water should be exceptionally clean to prevent infection.
If the injury hasn't healed over yet you can treat it with Betadine to disinfect it, after which the turt should be kept dry for a while to stop it washing off straight away. You can do this daily until it heals.

Will it grow back or stay 3/4 ? and thankyou

It probably won't grow back, not as it was anyway, it would be like you losing part of an ear or something as a child, it would heal and in time become less obvious but would never look quite the same.

shall i put salt in the water?

A small amount of aquarium salt in the water isn't a bad idea, I know some keepers use it all the time but I don't know in what quantity.

I have completely emptied the tank and cleaned it thoroughly and replaced everything and added salt.