After inspecting the support FAQs for Mavis v16, it seems probable that they had run into the network drive issue, since one of the faqs covered issues revealing that the location for the system.msy file had been moved into the "Documents and Settings" folder. Pity they never patched version 12.
I downloaded version 16 from a paid website - and received version 15 - unfortunately the site was in the US, so I guess trades description doesn't apply, but "Software Direct" are now on my shit-list. It also took 5 attempts to download - with Software Direct selling "download assurance" I had to jump through extra hoops to get my download.
Version 16 runs OK - but after all this I am kicking myself -- why didn't I search Open Source first ? I would have found "Tux Typing".
And Broderbund are on their last warning - this is the second time they sold me crap. The first was their genealogy package, proudly trumpeting all the data disks they bundled, but not revealing they are all US data. Not much use if you are researching UK ancestry and purchased the package in the UK.