Installation of a stone floor is a specialist trade. If you want the floor you have purchased to look as good as possible it is very important to employ a fixer with years of experience rather than asking a general builder to lay the floor. A specialist installer will be able to get the grout joints tighter than a general builder. A specialist installer will also be able to mix in the colours across the floor, ensure the stone floor is completely flat and that you get a finish, with the use of wax and sealer, that you are happy with.
Your retailer will be able to recommend specialist installers. If you decide to look elsewhere you need to look for evidence of a fixer who only lays stone floors and maybe specialises in English stone flooring, or Aged stone flooring and can provide recommendations and photos to back up his. You can find stone fixers via the internet or by consulting trade organisations such as the stone federation. It is always worth paying a little more for knowledge with the installation. You would not ask a plumber to do the electrics – therefore why would you use a builder to fix stone