Customisations, I was just loking in the wrong place before.
...
> If you are the owner of an account record in CRM & the account is
> re-assigned to someone else then all the activities (both open and history
> items!)are re-assigned to the new account owner. This is often not what is
> required - anyway that we can stop this so that re-assigning the account
> doesnt necessarily re-assign all activities related to it?
> Roger