First thing... spend a few bucks (or Pounds,in your case) on a device we, here in the U.S., call a Bevel Square, although I've seen it refferedto as a Bevel Gauge (like this:
http://hardwareaisle.thisoldhouse.com/2007/12/ how-i-use-my-sl.html ) and a nice size protractor (although for your job, you really don't need one.). Now, you're set to accurately measure the angle.
Once you've set the bevel square to the actual angle, transfer that angle to the
edge (not the face) of one of the base trim boards and cut it accordingly. Simply mark and cut the other, intersecting board in the opposite direction.
What you're trying to do now is set your table or radial arm saw to more that 45 degrees to cut on the face of your board, and, as you've discovered, it wont do that because of mechanical limitations on most such saws. However, the angle of the blade has almost an infinite amount of settings when cutting on the edge. You might find it easier to utilize a clamp on each side of the cut to firmly hold the upright board to the fence guide of your saw...
Best of luck!
By the way, you didn't say whether it's an
outside or
inside angle, but this method works either way.