Basically your sunroof is f**ked and it's time for a new one. I have one for sale if you're local to Suffolk but I bet you're not.
I had a similar problem with my first Mk3 Golf, and the only thing I could do was (from inside the car) push the sliders along with a flat headed screwdriver, although it does take some force

If this doesn't work, there's a possibility it could be getting stuck on the wind deflector thingy, which it was doing on my second Mk3 Golf, so I removed the wind deflector.
My third Mk3 thankfully doesn't have a sunroof. Although I'd like one sometimes.