As Des Browne strides up to me and demands to know who I am, the answer momentarily escapes me. It is on the tip of my tongue and will come to me in a minute, I am sure. But whoever I am, I suddenly realise, this close to Britain's combative Minister of Defence is not where I would like to be.

The occasion is a debate about the morality of replacing Trident, organised by one of Browne’s constituents, Father Joe Boland of St Matthew’s, Kilmarnock.

In opening the debate the priest had done a fine job of demolishing the pro-nuclear arguments. “The government says we don’t know what the world will be like in 30 years’ time, so we need nuclear weapons to keep us secure. But if nuclear weapons equal security, then every country in the world should have them.

“If Iran cannot have nuclear weapons when faced with an immediate threat from nuclear powers, what right have we, based on some future threat? It is sheer hypocrisy.”

Deterrence is doomed to failure, he points out, because it must work without error until the end of time. “We have been incredibly lucky so far.”

When Browne stands up he looks edgy but determined. The politics are complex, he says, and the morality unclear. We must resist more countries getting nuclear weapons, while those that already possess them disarm gradually and multi-laterally.

“That is the answer to the charge of hypocrisy. That is how we will achieve a nuclear-free world.”

In the meantime, deterrence is no more complicated than self-defence, he says. “I learned when I was young that if I didn’t want to fight I had to carry myself in a particular way. I had to generate a sense, on the streets of Scotland, that I could look after myself.”

He still does, I realise, after he has lost the vote and sees me taking notes of a conversation with one of his constituents, Sarah McKee. On the topic of hypocrisy, she is challenging him on accepting the minister of defence post, after attending a rally against the invasion of Iraq.

“I wasn’t there. You are wrong about that,” he tells her.

She starts to ask about his youthful support for CND - the Campaign for Nuclear Disarmament - when he sees my notebook and strides purposefully towards me. I identify myself and ask him a question – not the best question to ask a man who already feels under attack.

“I didn’t realise until today that you were brought up as a Catholic …” I begin. But he interrupts, moving closer.

“What's that got to do with anything?” he demands.

“I was wondering about Father Boland’s closing remarks,” I explain. “That in the end his opposition to replacing Trident springs from his Christianity. Jesus taught us to turn the other cheek, to love our neighbour. He would never have supported nuclear weapons.”

Browne responds that the churches disagree on war and nuclear weapons. Some support unilateral disarmament. Some don't. Then he goes on the attack.

“Are you a Christian?” he asks me.

“No,” I say. So he changes tack.

“Were you in favour of intervening in Kosovo?”

“Yes.”

"Bosnia?"

"Yes."

“What about Afghanistan?”

“Well…” I prevaricate.

“There you are…” he starts to say, but I misunderstand.

“Don’t give me that lawyer’s stuff about having to answer ‘yes’ or ‘no’," I say, annoyed. "Afghanistan was complex, militarily and politically.”

“Of course it was. That’s my point,” he replies. “International issues always are. It’s all very well saying ‘nuclear weapons are morally wrong’. But we live in a dangerous and complex world.”

True enough, I think to myself, casting my mind back 25 years to when I was a physicist with a young family, working for Rolls Royce & Associates in Derby. Working on the next generation propulsion system for the Navy’s nuclear submarines. The system that now propels the largest, most powerful submarines ever built in Britain.

The system that carries Trident.

But there is no more time for debate. The parish priest and the Minister of Defence have to leave. They are dining together this evening in the priest’s home.

“Nice talking to you,” Browne says, offering his hand.

I take it. His grip is firm but not crushing.

Deterrent.