This is a half n' half one as I've not fully done it yet, but incase anyone needs to I can advise most of the procedure.

Remove the plastic decor cap, iirc you will break it so you will need a new one or you should be able to glue it back in later,

At the bottom of the chrome shaft you will now have a ledge of sorts where the plastic cover once covered, push down hard on the ledge and the ledge will drop an inch or so, this should now make the tip knob loose.

Now the bit I'm not 100% on, if it will simply pull it off, but I suspect you have to either pull the chrome button out a bit, or push it in and pull the knob off.

Refitting the new unit, it should come with a clip holding the button in position, DO NOT REMOVE THE CLIP YET!

Push the knob on and hope for a click, pull the shaft back up and finally when you are satisfied it's on properly remove the clip.

I'm led to believe it only goes on once, so if you remove the clip early it can't locate properly and you've wasted a knob, hth.

Bumped for Stuart.