The fix:
With much effort the spring can be put back in. Here is how. First fully retract the belt on the spool. Next, the spring cover has a hole in the center which is covered by a foil sticker. Peal it off. It can be replaced. With the cover removed and in your right hand and the spring in your left, the spring hooks into the cover with the notch down and the spring coming out from the left side of the keeper. This is important because it appears that it should come out of the right side of the keeper. It look wrong because the spring wants to coil clockwise but it must be forced to coil anti-clockwise. Once coiled in the cover use needle nose pliers through the hole to tighten the spring about 4 or 5 turns and then while holding tension on the spring with the pliers fit the spring into the belt spool slot and at the same time fit the cover back. Lastly refit the foil covers for the hole. Sounds easy but it took me several hours. Most of the time was wasted because I had coiled the spring in the wrong direction and couldn't make any sense of how it could possibly work.

nick
'79 TR7 DHC
'76 TR7 FHC