The spring, if I remember correctly, rests on a "shelf"-like piece of plastic protruding from the cast plastic chassis.
If the edge of the shelf has become rounded, the spring will slip off. Everytime the spring slips off, the rounding will be smoother
and the problem worse.
I seem to remember that it's not very hard to grind it back to flat again, you may even be able to introduce a bit of inwards
slope to help keep the spring seated. Maybe a hobbyknife or similar will ease the task.
As an alternative, you could drill a hole and horizontally mount a screw with a fairly large head. The spring can then rest on
the screws stem and the head will block the spring from falling off the end of the screw.
Make sure to get the other end of the spring on to the tiny slider that rides in the groove at the "neck" of the hinge piece.
If you cannot find the tiny black slider piece, it will have fallen down into the Beocenter and can usually be found somewhere around
the large transformer, presumably caught in kilopoise grease from the lid mech.
Martin