I know the problem has been solved several times over, but this trick worked really well for me:
Bike on side stand, centre stand propped up to its closed position.
Attach a length of chain (fairly small link size) to the hook end of the spring via a D-shackle.
Lie on ground with head near back wheel.
Pull on chain to stretch spring and pop into hole in stand.
I was able to use both hands which made stretching the spring quite easy.
I was a bit concerned about how tricky it might be getting the hook into the hole, but it dropped straight in without a problem.
Once the preparation was done, the last step - fitting the spring - took all of about 3 seconds.