Baby Seat - Seatbelt Automatic Locking Not Working?

Hi all,

I've got a Mk3 Focus, and a new born baby... we have a baby seat that the rear seat belt of the Focus wraps around in various ways to hold it tight into the car, however this relies on the rear seat belt of the Focus engaging an automatic locking mode.

This mode is supposedly engaged by pulling the seat belt down and all the way out, then letting it retract back in (apparently coinciding with a clicking noise as the belt retracts) , and while in this mode the seat belt should only retract, and not allow any release of the tension, allowing you to pull the belt tight, and the seat belt keeping the tension.

Great, or so I thought until I tried to engage the locking mode, no dice, the seat belt just retracts or releases as under normal driving conditions.

I tried both left and right rear seat belts but can't get either to engage the locking mode, if one side worked and the other didn't i'd put it down to having one duff rear belt, but with both sides not working, I can only put it down to having a duff understanding of what I need to do to engage the locking mode.

Can anyone shed light on what trick is required to get the rear seat belt to go into the locking mode?

I'm grateful of any help.

Thanks all.

Phil