By Alex Kirby BBC News Online environment correspondent

On day 11 of Alex Kirby's diary from the World Summit on Sustainable Development in Johannesburg he asks why two key UK delegates have been "invisible".

The summit is in its final day, and it has just dawned on me: I haven't heard a word from two of the key people who came here from London.

Not only have I not heard them: I have seen them only once.

Who are these mystery figures? They are none less than the UK's Deputy Prime Minister, John Prescott, and its Environment Minister, Michael Meacher.

I glimpsed Mr Prescott one day legging it fast through the media centre.

I spotted Mr Meacher at a reception where he had come to represent the UK Government. And that has been all.

'The Invisible Men'

It would have been helpful to hear from them both, because they have huge experience of events like this.

I don't blame the Invisible Men... But it is not fanciful to say they have deliberately been kept away from us

Mr Prescott does his Jolly Jack Tar impression, and often manages to cajole deeply opposed groups to kiss and make up.

Mr Meacher knows more about the environment than most of the rest of the government put together and he cares about it, too.

I asked one of his minders for an interview with him.

As I am still waiting for an answer, there did not seem a lot of point in asking to see Mr Prescott as well.

There is a daily briefing for the British press (and anyone else who feels like turning up).

There are always helpful British officials to explain what's been going in the negotiations.

On three occasions there was also Margaret Beckett, the Environment Secretary (and therefore Mr Meacher's boss).

She is as helpful as she can be. But there has never been any sign at the briefings of the Invisible Men, Prescott and Meacher.

'Camera-shy'

Why not? They could have helped us to make sense of what was going on in a way that Mrs Beckett and her officials, excellent as they are, could not.

Yet they never appeared.

I don't blame the Invisible Men. They're usually happy enough to have a helpful word with any journalist who comes across them.

But it is not fanciful to say they have deliberately been kept away from us.

Before I left home to come here, there was a big political dust-up over reports that Mr Meacher was going to be dropped from the UK team in Johannesburg.

The government, apparently, was frightened of the tabloid press's reaction at the expense of sending out a large delegation, so it decided to leave the most crucial figure of them all at home.

It relented, and Mr Meacher did come here.

I am sure he and his invisible colleague have done invaluable work behind the scenes.

They could have done a lot in front of the cameras too, if they had been allowed to.

Why weren't they?