After I had lunch with the director-general of MI5, I understand the huge dilemma facing Labour - Nigel Nelson

I was rushing into the supermarket just as the spymaster was coming out and stopped just in time before we had a bodily crash.

What do you say to Britain’s top spook come upon unexpectedly so soon after a lunch? State secrets are obviously off the conversational agenda in such a public place.

So I settled for “hello”, repeated my thanks for a splendid meal (they really do serve up some delicious nosh in the Thames House boardroom), and we chatted about the weather.

The following week I relayed this surprise meeting to my regular contact in the Security Service. He was appalled. “What? he said. “You actually spoke to each other?”

“Well, yes,” I replied. “It would have seemed rude not to. And there wasn’t much choice given I almost knocked your boss over. Not speaking would have looked very odd.”

“Don’t ever do that again,” my contact instructed me. “And if you see me in a restaurant or on a bus don’t talk to me either. Pretend you don’t know me.”

As we didn’t frequent the same restaurants and I don’t use buses it was doubtful the situation would arise. But such is the strangeness of the spook world. Nothing is ever quite as it seems.

If you’re wondering what I was doing inside MI5 in the first place, there’s an innocent explanation. It’s important for MI5 and MI6, the Secret Intelligence Service, to brief a few select journalists from time to time.

Journos also stumble across stuff, and if that happens to be a serious national security emergency it helps to have some sort of personal relationship with the bod on the other end of the phone.

Neither organisation has a press office as such, but they do have public affairs departments for this purpose. Operational intelligence officers get seconded there to deal with the likes of me before returning to their undercover life again. Which is why their identities cannot be revealed.

I tell you all this to give some insight into why the mysterious case of the two men acquitted last month of being secret agents for China seems so peculiar. The duo, Christopher Cash and Christopher Berry, both deny the allegations.

But there is now political uproar over the case collapsing. Prosecutors say that was because evidence could not be obtained from the government labelling China a national security threat. The government says it’s the Tories' fault for not doing the labelling.

But because this is an espionage case I suspect there might be a lot more to it than the spooks wish us to know. It really is all smoke and mirrors with them.So we should cut ministers a bit of slack as they wrestle with questions they cannot answer because even they could be in the dark.

That has not put the Tories off and they sniff bad blood in the water. Kemi Badenoch is openly questioning whether there was government interference “to curry favour with the regime in China”.

This is a serious charge. Ministers, or special advisers or officials acting on behalf of ministers, should never stick their oars into the independent judicial process. Now Ms Badenoch has National Security Adviser Jonathan Powell in her sights.

But Security minister Dan Jarvis insists “he was not involved in any decisions about the substance of the evidence.”

The Tory leader won’t be deterred, and she may have another bite at the cherry when she faces Keir Starmer at PMQs this afternoon.

She said: “I suspect that (ministers) have decided that closer economic ties with China were more important than due process and our national security.”

And Lib Dem foreign affairs spokesman Calum Miller added: “The buck for this fiasco ultimately stops with the prime minister.”

This story has a way to go yet. Meanwhile, I’m happy to tell you I haven’t run into any more spies in Waitrose.

Although, as I’m sure you’ll understand, I wouldn’t tell you if I had.