It's a Multipla, you'll be an expert soon enough! (Pity, it's not optional)
Gently spring the lever assembly out of the floor trim and check that the cable end is still on and connected to the lever. If it still seems slack, don't be tempted to pull it.
If it is, check the lock mechanism in the boot, you may have to remove the trim to see this.
If they are both on pull the cable at the drivers end and get somone to check to see if the cable moves at the lock end, if it's really slack the cable probably busted somewhere and the cheapest option is to get another key cut just for the boot.
If it does move and pulling it with pliers operates the lock, you'll have to take up the slack, I can't remember if there's any tensioner for this or whether you'll need to crimp another end on the cable. Failing a crimp a connector out of an 15amp electrical 'chocolate box' connector does the job.
This is the reason for not pulling the cable until you've checked both ends are connected, re-ending it is a lot simpler when you haven't pulled the inner out of sight and then can't thread it back.
