John Kelly of the Washington Post features Squirrel Week each year, and he’s all over Mungo.
At the end of his most recent addition he clarifies where Mungo actually died:
But it turns out that when Mungo died, Twyford House was being renovated. The Shipleys were living at a place called Northwood House, 14 miles away in Chilbolton.
So that’s probably where Mungo was buried. Last summer I visited Northwood. Owner Jean Ward (Mrs.) graciously showed me around the house and the grounds. Alas, she’s never come across a squirrel tomb.