It is a complete nightmare to do with the rack in place but it can be done.. I was lucky that someone had fitted the rack riser to mine so the bolts werent too tight.. Il try and explain as best i can but im pretty rubbish at explaining stuff Ive put the number of the part from the diagram on it just to be sure what i mean. The way i done it was:
Remove steering arms from the centre bracket (13)
Remove the bolts (19) Think they 14mms, from the bracket. When these are removed the slider is now free to move side to side in its guide.
Snip the tie (5/12) that holds the rubber boot and turn the car to full lock to the left
If you stretch out the boot you will be able to get at the slider to pull it out. It needs pulled towards the front of the car to lift it out of the guide in the rack then pull it through the gap. I used long pliers to get at it.
Then when its out its just a reverse of what you did, its pretty fiddly/frustratin getting it fitted cause there is very little room. A bit of heat on the boot can help. But just push the new slider through the gap and into the guide. Cover it in grease and spray grease into the rack if you can.
Centre the steering when you have the guide sitting in the rack and then its a case of putting the steering arm brack on and moving it about until you get the bolts to line up
Hope this helps!