Ok, i can now report back. We have 10 mail servers, 9 of them are on the same CDO.dll as the main BES server. We have one development BES server and this has a different version to the Exchange.
With our deployment of O2K3Pro we installed the CDO components for outlook on the client, do you think the CDO.dll file on the client is relevant at all?
In any event I am going to bring all CDO.dll files up to the same version before doing anything else.