As I get closer to the date of the Great Bathroom Refit, I can bash
holes in things to find out what's inside without having to live with
the resulting mess for too long. I've just done exactly that with the
boxed-in soil stack:

http://pverdon.csoft.net/stuff/DSCN3581.JPG
http://pverdon.csoft.net/stuff/DSCN3582.JPG

My new layout will require the toilet waste to enter from a direction at
90º to the current one; more or less from where the picture was taken.

Do you think I'm likely to be able to simply swivel the branch on the
pipe, without having to break into the stack at all? Would make life a
hell of a lot easier if I could...

If not, I'm presumably going to have to pull everything upwards to
disassemble it. This is the highest fitting on the stack; it then goes
through the loft and out the roof. However, I'm worried that if I lift
it I'll break some sort of seal at roof level and cause a leak. This is
an early 90s house; is there a typical fitting that's used? (and does it
slide?)

Finally, when I bashed the hole in the side of the boxed in pipe, I
found it stuffed with fibreglass loft insulation. It doesn't serve any
immediately obvious purpose; is this normal and should I be aiming to
replace it when I rebuild?

Cheers,

Pete