I think he means Si not S1. I had troble with the boot lock before.

The only way to get into the boot through the cabin is to release the rear seats from the inside.

However you obivously cant do that.

you could however bend the rear part of the seat and reach in to get to the pull thing for the seats but it will reck them. (did this in a scrappy)

Maybe try unscrewing the seats though as if you were removing them.

I dont know if this helps - but i tried