Sorted - There is a line in the config.php that resolves the issue and gives me the 'continue' button missing from the above image:
'dbcollation' => 'utf8_unicode_ci', // MySQL has partial and full UTF-8
// support. If you wish to use partial UTF-8
// (three bytes) then set this option to
// 'utf8_unicode_ci', otherwise this option
// can be removed for MySQL (by default it will
// use 'utf8mb4_unicode_ci'. This option should
// be removed for all other databases.
By changing this to
'dbcollation' => 'utf8_unicode_ci'as above this has allowed me to complete the update.
Hope this helps others with the same issue.