Supporting flat roof beams, insulation, ventilation

[Sef1928172]

Hi

Victorian terraced house. Flat roof room and rear of property and shares an internal wall with the neighbouring house. (Recently purchased , so I don’t know the history of any maintenance/ work)

I have done a diy job of supporting some ceiling joists where a chimney stack had been removed. Previously there were rotting noggins in the gap where the breast was ( the stack is still in) but I have supported with new c16 to bridge the gap and sisterd to the joists that stop short of the wall.

unfortunately I couldn’t get new joists to span the entire room but am hoping bolting into the existing will be enough. I’ve also sisters some of the other ends that appeared slightly rotted (mostly out of caution).

My question is basically:

Can anyone advise if there are any glaring errors made here or will this do. The ceiling all seems to be well supported and it is a lot tidier than it used to be.

The next step will be to insulate. unfortunately I can’t see any ventilation at all (no air bricks of vents), so would a warm roof approach be best? Alternatively I can leave a 5cm gap and just hope for the best?

Any recommendations on the best approach for insulation? I am not too keen on doing a diy fitting of new vents (due to lack of ability)

Thanks in advance