Our Gestetner DSm645 gives Dark Cop...

Sounds to me like the drum needs replacing, and likely the developer too. There is probably a haze of toner still on the drum. You can check by opening the right side cover and looking at it. Is there toner on the drum (green cylinder). It may not be too obvious depending on how dark the copies are. GENTLY wipe a tissue across it after making several copies - it should be clean if the drum is functioning properly. If it is, it could be the fuser that is wearing out, but I'm leaning toward the drum. As I said, if you need to replace this, it is a good idea to do developer at the same time, because these machines tend to be a little sensitive.