A normal mortar mix 6:1:1 should work fine. If you are overcoating the existing mortaring the old stuff and surrounding tiles should be well wetted before applying the new - try to ensure you have enough depth to add at least 20mm thickness.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!