The Cave of Caerbannog, and the hovel where King Arthur was told of Tim the Enchanter in Scene 24 are close to each other roughly midway along the south side of Loch Tay.
The Scotland on Sunday newspaper of 20th August 2000 reported on a documentary being made by Michael Palin and Terry Jones for a new release of the movie on DVD in 2001. As part of the filming they were going to re visit the filming location of the cave. Unfortunately they, and the original location manager, had great difficulty finding the spot. Eventually after many false leads they found the cave again. The cave is actually an old abandoned copper mine which dates from the 19th century and belonged to the 2nd Marquis of Breadalbane.
In the documentary on filming locations on the Monty Python and the Holy Grail Special Edition DVD Michael Palin and Terry Jones visit Doune Castle, Loch Tay, Glen Coe, and Castle Stalker.