This is more of a "how-I-did-it" than a "how-to", just for interest and info. It is by no means the ONLY way to do it or even the CORRECT way to do it...it is just how I did it.
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I have had rust appearing around the mounting points of the skirts, on top of the sill, for ages now and this year it had got to a point where when I poked it with my finger it went straight thru. So time to sort it out as I didn't think it would pass MOT this time with that.
To start with removed the skirt. This is held on by 7 plastic push-clip type fixing along the top that you see when you open the door, 1 single screw in either end of the skirt and then about 5 screws underneath. On the P1s the rear end of the skirt has a removable end plate and there are metal brackets attached to the inside that mean you have to slide the skirt forwards to release from pins on the sill. P2s don't have this, nor the removable end plate.
Now in my case, I was extremely lucky and all screws just simply unscrewed giving me no hassle whatsoever, but a lot of you may not be this fortunate and the screws willbe corroded. A chisel or dot punch used with a hammer can get the turning if they give you grief.
pull the skirt off and expect to see it full of crap, years of mud etc, the perfect breeding ground for rust, being hel against the sill.

this pic actually shows it after a load of crap fell out on removal. Note, no clips on inside.
Now in my case I removed both doors when I was doing this to improve access, but my holes were very close to the hinges so I had to really to gain proper access. You do not have to do this.

Doors removed for access.