underneath (above?) the headlining - from the back
a zig-zag of mastic
and just some double sided stick above the screen (yes it was raining!)
I tried cutting the mastic with wire (bike brake cable proved sufficiently strong but very slow) but I gave in and attacked it with an 18" steel rule - with no rips or tears in the headlining! The mastic is the black stuff stuck to the inside of the roof in the a shape similar to an M.
Before refitting the cassette - which incidentally I couldn't fix but did manage to 'fix' it closed - I did apply a generous helping of silicone sealant to the underside to aid the seal.
Cassette is held in by a dozen or so bolts - see elsewhere in this thread for details.
a removed cassette without glass, or motor, and after my attempt to repair it:
I silicone sealed the headlining back to the roof (following the mastic)