Thank you all for your very helpful comments to my previous post. 

Can I ask you please what tools you use when starting a project, e.g. to 
help with defining what the application will do in OO terms and in 
defining classes?  I know there's plenty of software out there for this 
kind of thing, but I have no experience of it and would like to know 
what real live Squeakers use.
Thanks,
Tony.