Strangely enough, mine is sagging also. It's been on my to-do list for ages.
Today I fixed it. Mines a '93 defender, it has the roof ribs internal, not on the outside like the series.
I took out the stupid plastic plugs (don't know of they were original), at the rear of the headlining, marked the position of the three ribs and then put the rear plastic plugs back in to secure the lining, I then drilled 3 holes across the front (ish) of the lining, (just inside the 'sunroof' indentation) and then drilled another three further back and used the expanding type plastic plugs (the ones where you push the plug in and then push the little centre plastic bit to expand them),to pull the headlining back up to the roof again.
Just remember to make sure you hit the internal ribs and not drill a hole staright through the roof.