Stepping stones - Mortar/bedding mix

[Staticbird]

What mortar or bedding mix would you recommend for stepping stones? They're limestone and vary in size from 600x600 to 300x300. They're surrounded by 20ml chippings but sit slightly higher so the bottom of the flag is level with the top of the chippings (trying to minimise the stones just getting covered in chippings). We're currently relaying them because they all rocked. There's no sub-base at the moment - they were laid straight onto soil (using mortar). Would you recommend any sub-base at all? The previous guy used dot and dab technique - would not using that deal with the rocking on its own? What would be the best way to create a solid, long-lasting bed?

Thanks very much.

[lutonlagerlout]

dig out deep enough and lay them straight onto 100mm of wettish concrete,solid bed of course
LLL