I got to thinking and remembered that I have had the problem too, and I'm sure I fixed it myself. The mounting bracket to the body cracked I think, casuing the strut to become ineffective and the boot lid to droop dangerously low. To fix it (it was the L/H side) I reckon I removed the rear seat trim down the left side and unbolted the bracket and after disconnecting the strut. Then I'm sure I bent things back into place, lit up the red and black (oxy welder) and bronzed it up. I reinstalled it and it's never looked like playing up since. That's what I did.
Remove the rear seat side bolster, and find out where the bracket has broken off, most likely the spot weld has let go. take the bracket off the strut and drill the spot weld out with a 6.5-7.0mm drill bit, drill two holes there and two holes corresponding, in the bracket and just bolt it back into placewith a couple of 6mm flange nuts and bolts.