I'd like to insulate the bathroom. The walls will have to wait until I
re-furb the whole thing properly, but I thought the ceiling would be a
good place to start.
The ceiling is in three sections, following the shape of the rafters. (a
slopey bit each side and a horizontal bit under the collar beam in the
middle. I bet there's a name for that shape, but I can't find it.) The
sloped sections should be trivial(-ish): Just PIR PB following the shape.
The horizontal section has a space above and, on the (internal) end
wall, a hatch gives access to the void above the corridor outside and (I
thought) the space above this ceiling.

Clambering up there, I find this 'internal gable' end is bricked up,
though a helpful electrician has removed a couple of bricks so I can see
in. A lot of the space (which is smaller than I expected but I should
be able to crawl into if I had access) is filled with the debris of a
historic re-roofing job, that will have to come out before insulating.

So my problem is access and I could do one of the following:

Cut more bricks out of the gable until I can get in. My reservation is
that the bricks are presumably there for a reason, so I would have to do
something fancy to maintain the structural strength. (Though it's not
clear why the rafters don't do that.)

Insulate with PIR PB like the rest, on the underside. The problem here
is that the access hatch is hard against the ceiling and I don't think
it would be big enough to use if I make it significantly shorter. It
would also look a bit weird if I stopped the insulation layer short to
allow for the hatch. If I put a false ceiling below the level of the
hatch, it would make a pretty oppressively low ceiling.

Take the roof tiles off and insulate everything from above. Access from
one side is pretty good, although I would need scaffolding to take the
other side off. Main problem is it could be a very bad time of year to
start doing that, especially when I only have weekends. It also seems a
bit OTT just to get some insulation in.

I can't help feeling I'm missing an easier solution (apart from not
bothering!). I'm banking on someone here being able to provide me with
some inspiration or experience of this.