Sorry for the late reply. You will need form AN currently £655 and any children will require form MN1 at £400 as far as I can tell. The nationality checking service merely checks that all your documents are in order. Be sure to enclose original marriage certificate and a copy even though this is not specifically asked for in the documentation list. I feel that using the nationality checking service definitely speeded up the application as it was processed in 16 days! The ceremony (a farce really) was around a month later. We had to use the service as we wanted to go abroad on holiday and by using the nat service allowed us to retain the passports.