In Assembly workbench you can use Tools - Generate CATPart from CATProduct. This will do a single CATPart which will help you to go forward with your method.
Thanks for the replies, very useful and informative.
Problem I have is many of the parts are hollow, so subtracting the volume might not give me the actual air displacement.
Has anyone found a good way of getting Catia to only account for volumes bounded by external surface, in order to work out the displacement in a fluid?