Thanks both.
For clarity to anyone having the same problem, here's what is says in the Release Notes
If you see 404 errors after upgrade please either delete the .htaccess file so that it can automatically be regenerated or find;
RewriteRule ^(.*)\.html?$ index.php?seo_path=$1 [L,QSA]
and replace with
RewriteRule ^(.*)?$ index.php?seo_path=$1 [L,QSA]
Now working like a dream!