Entering the tunnel, we pass a small shrine above our heads which contains a statue of St Barbara, the patron saint of tunnellers. It is traditional to have such shrines on every tunnel site.

Graphs can be prepared on blue-lined graph paper and then photographed or traced.

But the film is largely redeemed by an unexpected emotional resonance befitting a Steven Spielberg production.

In the following year blight appeared again, but at another point, and after cutting it out I put on tanglefoot, simply because I happened to have some with me when passing the tree.