Few Londoners realise that they share an estuary with a secret military island that once harboured the UK’s atomic weapons research programme.

Access to the 6,000-acre isle of Foulness is strictly controlled by the Ministry of Defence. In the Sixties and Seventies, it was used for testing beryllium shells, and now it has 150 civilian residents – though it’s only open to the public on the first Sunday of each month between April and October.

Under the capital is another curiosity: a network of tunnels, deep beneath the Underground in Holborn. They were constructed as the last redoubt of the British state in the event of a Nazi invasion; now they’re owned by BT.

I investigated, explored and trespassed my way through strange places like these in the course of writing my book, Who Owns England? Behind the question in the book’s title lies this country’s oldest and best-kept secret: a hidden history of land ownership that stretches back to the Domesday Book.

I first became interested in trying to uncover who owns England when I realised how fiendishly difficult it was to find out. What was there to hide, I wondered?