It is easier to take the tub out (that means the roll bar has to come off!) as trying to get to the seat belt reels through the speaker holes is OK on the way out but nigh on impossible on the rebuild. You need to have double jointed fingers to get the bolt in place even with full access.
For information, I had the typical lazy Fiesta seatbelt on the driver's side only, even from new, and in desperation I rewound the spring (not for the faint hearted!) such that the belt will just but only just go to full extension. This was accompanied by a very annoying tendency not to extend at all when the car was either horizontal or pointing downwards. After playing with the angle of the belt, this was improved but not cured until all of a sudden it appeared to go away.
When the seat belt reels were off the car, examination of the mechanism showed the lock-up was due to the ball falling off its seat (as it is supposed to do) at the slightest movement and nothing else. There was certainly no dirt in the system as the whole thing was blown out with an air line so this was unfortunately somewhat inconclusive and as to why it suddenly got better again there is no clue.
My final modification was to move the intermediate anchors that hold the belt at shoulder height from the top of the tub onto the roll bar, which, when combined with a cut in the tub such that the belts go up flat from the reels to the intermediate anchor, made the whole thing a lot smoother and the drivers side now rewinds properly.