Ok this is now a general question as I managed to import the user table to the database.
Unfortunately it appears that whilst the users all show in the database they do not show in the CMS. Digging a bit deeper it seems they are related to the elements table as each appear to have an entry with a type User.
So my question now is how can you import your existing user list into a new craft set up without breaking it or anything else. With over 10000 users this is not one to be done manually.
Any help appreciated.