I have just contacted Norfolk Line Ferries (who we used last year) to ask about sailing times.

Going out Friday 28th May, ferries start at 2.00am and are every 2 hours after that (so 4.00am - 6.00am - 8.00am etc)

Returning either Monday 31st May or Tuesday 1st June, it is exactly the same timetable from Dunkirk and they run up until midnight.

Next question is, are we going to try and do a group booking? Or are we doing our own individual bookings? There will be a small saving to be had by booking as a group, but then there is the problem of one person paying and everyone else getting that person the money.