'Inside, if you please,. sir'

'I don't see why.. '

But I was already in; the doors thumped shut and we were in a bare brightly lit concrete chamber. Another pair of steel doors lay ahead. We might have been on a Mars science-fiction set, except for a Navy guard sitting facing us behind a bullet-proof window. His telephone was yellow.

'The doors ahead can't open before those behind are dosed,' said my guide conversationally. 'And that can't be done without that bloke's say-so. Security's a hundred per cent. Those doors can also take a direct hit from a 500-kilo bomb, without a blink. This space becomes an airlock in the event of a nuclear attack.'

'If you tell me this is Hitler's bunker and you're Eva Braun. I'll believe you.'

`No dolls here, more's the pity. Males, rugged as they come. Hand-picked. Zip-lips.'

'Listen! Before I move another bloody step..

'You'll have to now, sir. Can't stop here.'

The huge doors rumbled open on their runners. Several concrete passages radiated on the far side. I was propelled into one of them and my escort fell into step with me.

'Silvermine,' he said. 'It's called Silvermine. They found the metal here in 1687, I think it was.'

'I hear your words but I don't know what the devil you're talking about! Silvermine! Nuclear attack! Airlock!'

'It's new since your day, sir. It's the Navy's operational headquarters. It's sunk into the side of the mountain, storeys deep..

'The hospital, man! My mother's dying!'

'The C-in-C will tell you about that, sir. We're almost at his office.'

We turned into a side tunnel; a small pair of doors, now open, could seal off the passage. Nevertheless, we faced the same elaborate security paraphernalia. When finally we were admitted to an ante-room it looked less like outer space: I saw an ordinary office desk, a typewriter, filing cabinets and wall-to-wall carpeting.'Hello, Godfrey,' said my escort. Well, here he is.'