Will it tilt forward with the engine off and doors open?
There is a hook that pulls forward over a peg that locks the seat back in position, it is vacuum operated from a diaphragm under the center part of the front seat at the front. There is a cable operated [from diaphragm] hook each side at the back of the seat, chances are the hook is not returning under the action of its spring, probably because the seat is not sitting hard down on its stop and the peg is holding the hook in place or possibly because the spring is broken or has slipped.
Is it both sides?
You can get better access to one side at least if you take the outer plastic side molding off the seat, they just clip on and off - you should be able to see the hook and peg. whether you need to remove the seat to get access to the inboard one you will come to understand.
Today I had a better look and removed the plastic cover to the side. Found the hook and pulled it quite easily which released the seat. Seat tilts forward and once back fully the hook grabs it again its locked. The button does nothing and feels like its doing nothing (although so does the other side but that one works)
So looks like this may need to be a seat removal job
So the problem is the hook will not on it's own release from the peg? Things to check are,
1] Spring not functioning properly - i.e. ends of spring dislodged or spring broken,
2] Slightly bent peg or hook causing them to be misaligned slightly,
3] Seat back not seating properly on its stop on the seat base, raising the position of the peg slightly so that the hook cannot release from it.
All the button does is release the vacuum from the diaphragm and allow it to relax the tension on the cables to the hook. The springs are the device that returns the hook to its rest position.