garythetiler

first thing you must be sure of is that the york stone is solid and free of any movement as you cant see whats underneath the york stone it might be better to take it up and put a concrete base down, you will get good money for reclaimed york stone ,if you cant do this and you are sure there is no movement i would lay an uncoupling membrane, dpc or crack isolation membrane of your choice then you will need to screed over with sharp sand and cement mixed to a ratio of 4 sand 1 cement mixed to a semmi dry consistancy so when you grasp the mortar in you hand it compacts and doesnt fall apart i would put wire mesh reinforcing through the screed remember to create a fall running away from the house your ideal bed depth should be between 38-50mm leave this to cure (approx 1 week per inch thickness )then tile with exterior grade spf cement based adhesive and grout with flex grout