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Hello and welcome to the best of today. I'm Nick Robinson.

At this time of year, the Today program has guest editors.

People we invite not to take control of the whole program.

That's our job here at the BBC.

But people who can bring a different agenda, a different perspective,

people who can reach guests that we would perhaps struggle to reach on our own. Today, our guest

editor is Sir Jeremy Fleming, the director of GCHQ. Now, you'll probably have heard the name

GCHQ, but if you're like most people, you won't know heard the name GCHQ,

but if you're like most people, you won't know a great deal about it.

This, despite the fact that it is the largest intelligence agency,

bigger than its better-known cousins, MI5 and MI6.

Originally, GCHQ was called the government code and cyber school.

Its job was military signals intelligence, and it began doing that work before the First World War.

Now it gathers intelligence largely over the internet and mobile networks, not just on the

country's potential enemies and adversaries,

but also on criminals and terrorists too.

It works to protect the information and the information systems used by us all,

but in particular by government, big business and the armed forces.

Around about 5,000 people now work at GCHQ's headquarters on the outskirts of Cheltenham

in what is known as the donut, a huge ring of a building filled with people who can't, by

law, tell their friends and family what they do.

...