We do not know under what conditions you are keeping your plant.
Temp. Watering. Sunlight or artificial light.

Not that I am good at this sort of thing myself, I have several plants that are doing very well and a few that are not...;

Maybe a weak general purpose fertilizer would help?
Find out what root mealie bugs (Google) look like and check the roots.

Sorry, it's not good to advocate fertilizer for plants that aren't doing well; it's not like medicine.

Tho' I grow mostly succulents & few cacti, the important care is the same, they must have fast draining mix which that likely is not & the plant looks like it might have been overwatered as well. Pls. read around on this forum for discussions & info on better mixes, many such discussions.

can i save it by cutting the top of before it gets to the rot ,am also in the process of making a new draining mix for my plans .

been keeping in south facing window the weather isn't really hot or humid ,my house id quite moist and can have condensation at times ,in my old house during the summer it did well in a south facing window with lots of sunshine but not direct ,bit confused with what type of condition they like in the winter as i live in rainy wales

had a look to see if my cacti has root rot ,is it that bad ,what shall i do next ?

I think your cactus is Stetsonia coryne. I have one and have ignored it for most of the 6 years or so I have had it...

Your cactus doesn't have root rot, but beyond that, don't know what is causing the yellowing.

Christopher

thnakx for the feed back,nice cactus you got there,ah right most probaly a goner then :(

i get the gist of what am supposed to do than for help, just to make shire before i do anything is this correct in the photo

fallow up picture