I would have thought you'd need to bed them down in sand - and level them as you lay them, or you may well end up with trip hazards!

I agree with spritzhenry also you may have problems drainage as well

I agree with all of the above. My suggestion would be to break up the concrete first!

I hesitated to say that, Ian - such a big job... so this guest may not be able to do it, but starting afresh would be the best option, of course.

Which ever way guest goes about it Spritz, the most important thing is to make sure they are laid on a good solid base. Perhaps if he was to lay them on a bed of mortar (cement and sand) this would alleviate the drainage problem? Just make sure that you get the run off right. (A very slight difference in height between one side and the other)

Guest, if you know someone "in the trade" then quiz them for details!