Tried to fold down the 60 of the 60/40 folding rear seat at the weekend. The release button wont budge. Stuck solid, will not depress. (no its not locked with the key

I''m assuming something broke when the backrest was last put into the up position.
How do I go about fixing it ?
Do I attack this from the rear in the boot space or do I somehow need to remove the seats ?
Thanks in advance for any assistance.