Avoiding cracking  when turning olive has eluded me. What is the method for avoiding this? Turning wet, sealing the wood and drying slowly etc. have all failed. Suggestions please.

Along with the David Marks information I'll just add this.
First of all, you're not alone, this is a common occurrence.
SLOW drying is really the only way, even then there is the chance of checking or small cracks.
Olive is a fruit tree with many branches and dense grain that is typically anything but straight. This imparts internal tensions into almost every section of the tree.
Also, out here where we live (DM isn't too far away) the temperature and humidity changes drastically everyday. The winter is a bit more stable in terms of weather, without 30+ degree temperature swings and 50 point humidity swings.
I have had totally wax sealed California Olive crack. 
I don't know the waste percentage with Olive wood but I can only guess it's very high.

The majority of pieces I've seen around the area at gift shops etc. are typically small and a bit thick or even "clunky". This seems to say as much about the wood as it does the turner.
If you can manage to tame Olive wood, you're on to something.
Good Luck with the DNA