Aaaa ha!
Thanks for your quick reply – yes that was the problem.
I think installing W3TC Browser Cache after MultiSite may have rewitten my .htaccess file.
Adding the line
# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]
into
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
worked.
Thanks again.