Smurf - make your own mitre-box. Two bits of board, 20mm higher than your skirting, will form the uprights, another piece of timber the width of your skirting as a spacer, screwed at the base between the uprights - there you've made the box. Now the tricky part - go to your school geometry set and select a 45deg set square (or toolbox to get your mitre square), strike a 45deg line across the top face of the box; next, with your set square (or carpenter's try square) strike 2 vertical lines down the front & rear faces of the box. These are the lines you will cut down.
How to cut - never try to cut along more than 2 lines at the same time. Fix the box high up in a bench vice (or screw in down onto a big piece of board for support), slip a bit of timber in the box to stop the front and back collapsing in the vice. Next, take your panel saw (or tenon saw to start) and start you cut on the back corner gradually dropping the angle of attack to the horizontal in-line with the 45deg lines. Your aim here is to have a horizontal cut about 10mm deep.
Next, using this 10mm deep cut as a guide, cut down the back face on the line - remember the 2 line rule - by tilting the saw. When you've got down the back face you can start to cut the front one; you now only have to observe the front line as the back line is acting as a guide.
This angled sawing method is the classic way to cut tenons, etc.
Or you could make a mitre shooting board ... instructions on request.
