Today my UK Ford Focus hatchback failed it's annual 'safety' MOT. One
failure was that one of the seat belts wasn't working. It was the belt
installed in the rear seat for the middle rear seat passenger. It was
stuck fast i.e. it couldn't be pulled out of the seat back.

The person who performed the test said it was a common occurrence and
not only on Ford cars. Often a hefty thump to back of the seat would fix
it – it would free up the stuck spring mechanism. In this case he had
already tried this and it hadn't worked.

Sure enough when I picked up the car, and the failure certificate, the
belt was well and truly stuck.  After 10 minutes attempting to see if I
could remove the back of the seat to expose the mechanism the belt
suddenly started working :)

Within the hour I was back at the MOT station with the seat belt and
some other minor problems fixed and obtained my pass certificate.

Later I observed that if the seat back, which folds forward, wasn't
properly upright and in the 'locked' position that the seat belt was
stuck. If the seat back lock was correctly made the belt was free and
operated normally. If the lock was only partially made the seat belt was
again stuck.  I assume that in my case the belt had initially started
working again when I was messing around with the seat and I had
re-seated it in the bottom pivot point and locked it in position
correctly.

Is there an interlocking mechanism built into the back seat that
prevents the middle seat belt from being used if the seat isn't fully
locked into position or have I a potential problem for the future?