I'd think either a mortar or concrete bed. However, will the Yorkshire stone stick to the side with these or do I need something else as I can't see either getting much suction on the stone?
I have laid plenty of sandstone and limestone slabs as steps and mortar is fine. Use a strongish mix with building sand though for the extra grip. 3 or 4:1
Yorkstone has plenty more suction than pressed concrete flags and they grip mortar ok for many steps.
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