Thanks to All. I've managed to get filtering to work and setup folder structures for each of 2 email identities used by wifey. Now she gets nothing in 'New Mail' and has to look at the unread color flag in the import folders, then delete or move mail to a relevant folder. At least if she doesn't do that, all her mail is now in 2 folders instead of one.
One question: Are the filtering options executed hierarchical? i.e if I put a global filter at the start of the list e.g to delete spam mail, is that mail filtered first then those further down? Conversely if a global filter is put at the end of the list is that filtered last?