Seat Belt Problem

[Pressurized]

Does anyone know of any causes and hopefully fixes for the seat belt having a fault where, from time-to-time, it's hard to unwind?

Most of the time the driver's belt can be unwound and put on perfectly fine but sometimes it keeps locking even when it's pulled very gently and slowly. When this happens it can take a while to sort - you have to keep winding it back slightly to release the inertia lock then carefully stealing a few more inches before it locks again.