Thanks Highfall, some good advice in there. This issue is now 3 months old and we are no nearer to a solution by the looks of it. I do now have quite a good understanding of calendar issues and how it all hangs together as we are now being supported at the top tier of MS who work with the source code. I have learnt that the is error message is caused by a discrepancy in the PR_CHANGE_KEY which is a hidden property on a calendar item. In order to see this key you need a tool called MFCMAPI which you can then use to see all properties of items and then ultimately use to determine if there is a "ghost" process changing the calendar item in the background. We have now figured out that our items change as much as three times without anything being physically done to an item so we are now trying to track down the origin.
I will keep you all informed but thanks again to all for your input, it's all valuable all the time!