This is how I have just fixed both of my weak retracting seat belts. Remove just the reel from the chassis and sit with it on your lap in the car. Because reels differ, you need to identify which end cap work on. Press each of the plastic end caps to your ear in turn, and listen for the one that 'ratchets' when your pull the belt out and back. You DON'T touch this one.
The other cap should have 3 plastic pins which retain quartered plastic fixings which hold the cap onto the metal frame. Carefully push the pin partially back to release the quartered fixing and allow the cap to be prised from the frame. BE VERY CAREFUL TO ONLY JUST RELEASE THE CAP, if you let it go, the recoil spring could fly out ! Please watch youtube - 'Rewinding a Seat Belt Return Spring and What NOT To Do' !!
Now, VERY carefully and maintaining a constant pressure motion, turn the cap anti-clockwise (in my case 3 turns) and pop the cap back on. Push the pins back in to secure. It's quite simple with care and patience. That worked for me, but if the retract is still weak, I imagine a few more turn might work ?
Disclaimer :- YOU ARE WORKING ON SAFETY EQUIPMENT. I am happy with my work, but I CANNOT BE RESPONABLE FOR YOU ATTEMPTING THE ABOVE FIX.
Having saggy seat belts on my 2011 plus 4, I felt motivated after reading this to have a go! Fat chance! I recall from a factory visit seeing the reels mounted on the woodwork, prior to panelwork. Removing the spare wheel, I can see right up in the corners, that the reels sit in a steel cradle. Due to positioning of woodscrews, the cradle would be impossible to remove without removing the body panels! Possibly, the reel could be removed if it is only held by the single nut and bolt that I can see but barely touch with a finger tip. The nut is even recessed into the wooden crossmember to make it more difficult. It might be about 17 mm or so, but there is no room to get a spanner or socket to even try.
Yet another example of “fine Morgan craftsmanship and design”.
Has anyone had a go at this and succeeded?
I think it does not bode well for any owner of models built in this era. You might put up with sloppy belts, but break a recoil spring or even having to replace a frayed belt for the MOT, might mean serious bodywork disturbance. I despair!
