Cheers Chris, came off with what I thought was excessive force, this skirt won't be falling off in a hurry under normal circumstances!
So in summary for anyone else doing this.
1) Remove about 4 screws on the top edge hidden under the door seal
2) Remove about 7 screws on the bottom edge near the sill lip
3) Remove a screw at each end.
4) Tug very hard starting at the front end of the skirt, there are 5 clips in total, one at the front end, 2 around the door (which came off easily) and 2 at the rear end which were the hardest to remove.
5) fall over when the skirt comes off suddenly.
Glad I removed them because there appears to be some surface rust appearing on the sill which I will clean up and repaint before fitting. I'm also going to spray ditrinol/waxoyl in the sills for some added protection.