It would appear no fixed 1st/2nd sittings for dinner except on the Gala night here is the extract from the website:
Dining - waiter service
• The Compass Rose Restaurant: Serving breakfast, lunch and a six-course dinner. All meals are served as open seating so you can dine when and where you please.
• Sirocco's: Intimate à la carte restaurant offering the ultimate dining experience. A cover charge applies and onboard reservations are required.
• Gala night: Weekly Captain's Cocktail Party and Gala Dinner. Set dining times apply - early or late sittings are allocated on arrival.
Thanks for info its been very helpful WE DIDNT WANT TO BE TIED TO MEALS!