This happened to me a while ago and I think I posted the fix.
Basically what usually happens is that the release mechanism seizes then sticks across the lock so when you close the hatch it breaks the metal connecting the solenoid to the lock. You need to crawl in from the back. Unscrew the plastic cover (torx screws on mine) You will then be able to see the lock. There is an obvious bit to press to release the lock.
You will need a new lock mechanism but any 156 boot lock mechanism will do, even the 4 door.
If you are still struggling let me know and I will try to take some photos.
"When you sit in a Focus RS or a Golf R32 itís like sitting in a commercial for Lynx aftershave. When you sit in an 147 GTA itís like sitting in a Venetianís hand-made suitcase." J.Clarkson 2003.
Current fleet: 156 Sportwagon jtd 16V (facelift) 2.4 Spider, Leon Cupra SC
Projects: Spider S4, Matra Murena 2.2