Hello,
as regards 404 problem, if your site (probably testing instance?) is now installed in "
www.nav-e-gate4less.co.uk/Joomla_V1_5/", make sure that your RewriteBase in .htaccess file is changed to /Joomla_V1_5.
As regards the VirtueMart JoomSEF extension, similarily as when migrating from JoomSEF 2.x to 3.x, you need to purchase the new version. JoomSEF + its extensions for Joomla 1.0 and 1.5 are quite different and moreless written from scratch again, so that is why the online upgrade policy does not apply to this migration.