I have a Mondeo, the seatbelt won't work unless the seat is fully on the catch and you have to properly slam the seat back onto the second catch to make the seat belt work, the seat catch is in two stages and you need to make sure it's on the 2nd catch. Hope you get it sorted.
this is true i work in a mot garage and come across this problem quite often (the ford focus is the same iirc) make sure the seat is secure and locked fully and it should work
