Yes, it should be 1.1m.
Technically, it shouldn't be allowed as the guidance only recommends up to 4 levels (including ground) if there is a single staircase and, I guess, this is a fifth level. If I have read it wrong and this is the fourth level then that's ok. Even if it is the fifth level it should be ok provided that the stairs are lobbied at all levels (but not at the rooftop). As the top floor is over 18m the common alternative to lobbying of relying on a single fire door and the L2 alarm system will not really be adequate.
Technically, with a floor over 18m you should have a firefighting shaft with firefighting lift!! But you can argue that away by virtue of the fact that this is simply a seating area with a negligible fire loading and so there should never be a significant fire at rooftop level.
Similar argument for dispensing with the need for a fire door to the roof from the stairs - also this argument is supplemented by the fact that any heat and smoke will disperse upwards and not attack the door to the stairs.
Definitely less than 60 seats and maybe less depending upon the capacity of the staircase and the maximum populations on the other floors. Travel distance no more than 18m to the storey exit.
I guess the area might be inaccessible to people with impaired mobility but, if not, then you wil have to plan for their escape.
