Gordon Carrera

Early on, MI5 says, oh, he's an unreliable, mercenary, petty criminal.

And although well-placed, has frequently failed to give advance warnings of PYRA activities, and a very large proportion of plans on which he reported seem to have come to nothing.

But at the same time, we do know that both MI5 and RUC Special Brands try and take him over from the Frou.

So despite some of those early concerns from MI5 at various points, they basically try and steal him, lure him, pull him over from the Frou, because I think that's all a sign really of how valuable he is.

Yeah, so he is number two, as we said, in the internal security unit.

But what that offers is, first of all, the fact that he's just a senior IRA man.

So he knows who's who, who's up, who's down, who's meeting.

He's got the gossip.

There's talk that he's able to drive around with other senior figures in his car, with his car being bugged, is one of the claims, for instance.

He's also got a sense of who's joining, who's in.

The ISU is actually described as the Clapham Junction of the IRA.

Now, David, do you know what that reference means?

Maybe not.

Clapham Junction, which is actually really in Battersea rather than in Clapham.

Long story.

It's very confusing.

The reason is that lots of train lines going into London stations go through Clapham Junction.

So it's actually for a long time was the busiest, I think, train station, not just in Britain, but in Europe or something like that, because you've got trains going into Victoria and Waterloo.

They all go through Clapham Junction.

And that's the analogy with the ISU, which is that lots of activity within the IRA went through the ISU.