OK, so these pictures may not be the most exciting, but we dutifully followed the bright orange signs dotted around Pierrefonds and found the wooded area where they'd been filming the last scene of the finale.
They certainly weren't shy of spoilers as the signs all proudly stated "Excalibur"! We were actually very lucky as about five minutes after we found it the crew guys drove up to take away the prop rocks - they actually had three to choose from, as I recall.
I'm not sure exactly where the place is - we had to walk along the path for quite a way - but I've made my best guesstimate on this map of Pierrefonds (the black dot up beyond the church).
I wasn't sure what tag to use for this as it wasn't actually at the Chateau itself, but if you want me to add that one I will.