The cable release in the boot is on a spring.

If you get some small nose pliers between the seat and the parcel shelf then you may be able to unhook it from the catch.

Alternatively you could try a small flathead screwdriver to jimmy the catch. Again between the seat and parcel shelf.

If you know someone with a saph then look to see how the catch works and where to lever it.

Once it's open then remove the 'D' catch on the back of the seat so you can still get access without it clipping back in.