Bathroom ceiling insulation

[Super_Paulie]

my bathroom ceiling (first floor) is a mess, looks like its been a separate toilet and bathroom at one point and then converted into a toilet, bathroom and corridor. Now its all 1 room the ceilings are all over the place so ive lowered it down with C16 to get me a level surface. Its got a loft above with fibreglass insulation between, plastering lath is still here, and its going to stay there as well as its manky above it.

 

My question is, should i fill this newly created dropped-ceiling void with tight fitting PIR and tape the timbers for my vapor control? The C16 is 38mm but packed out in places, often adding 25mm here and there to compensate the old ceiling, its that wonky. I could get PIR in there but it will leave dips and voids above it in places, any thoughts on mixed materials as i could pack it with 25mm fibreglass and then 25mm PIR level with the new, lowered ceiling timbers. Ceiling has a vent with an inline fan in the loft existing through the roof, room is small, 2.5m sq.