How would you know if one of your colleagues was a murderer? When police announced the man they’d arrested for multiple horrific murders was Dennis Nilsen, many of his former colleagues — including me — were amazed, but perhaps not completely incredulous.

Des worked with me at the Hotel and Catering Jobcentre in 1980 and he was unquestionably odd. My wife recalled him saying in the office one day: ‘You know it would be really easy to pick up some rootless young man in a bar and knock them off. Who’d notice? Who’d care?’

For his colleagues it was just another rant, a weird take on his continuing critique of Thatcher’s Britain. ‘Oh shut up, Des!’, they no doubt thought.

David Tennant — whose portrayal of Nilsen in the recent ITV drama Des was faultless — described Des in an interview as ‘actually rather boring’. But he wasn’t bland. Des had strong views on all sorts of things. An open-plan office gave him a captive audience to share his thoughts with.

I used to joke that Des took up so much of my time as his manager I really ought to have him written into my job description. In the ITV drama, he congratulates Brian Masters on his book Killing For Company, then tosses him a notebook filled with his comments. It’s an experience I was familiar with. Des was the local rep for the junior civil servants’ union and I had to read his endless stream of handwritten notes spelling out his complaints at length.

Relations between us weren’t always good. I once informed Des that I would not allow him to work with the public: ‘Your manner in relationships with your colleagues is usually outspoken and often overbearing. I am concerned that… your manner with the public might cause offence,’ I wrote to him. His reaction was furious; he threatened to sue me for libel, though he backed down in the end, as he usually did.

If Des was a difficult person to manage, he could also be funny. ‘This office,’ he once told me, ‘is like a Land Rover stuck in the sand. Every now and then “management” (he always talked about us in inverted commas) turns the key, and the wheels spin, and nothing changes. Then we get a shiny new management trainee who says: “I know what we should do; let’s paint it yellow!”’

It would be wrong to say Des was a bad employee. ‘We’ve had our differences in the past,’ I told him in a letter some time later. ‘And we’ve had our differences this week. But I stand by my belief that you are “fitted for promotion”.’ Des proved me right: he got his promotion and moved to Edgware Road Jobcentre, where he was working when he was arrested.

Were the warning signs there? I’m not convinced they were. Des never gave me the creeps; I never felt uncomfortable in his presence. There is no doubt that he was a pain in the neck. He was confrontational and provocative in his opinions and eager to challenge authority. It could be said, though, that his willingness to over-share clashed with his reluctance to talk about himself.

In the time I spent working with him, I never socialised with him. I did visit his ground-floor flat in Muswell Hill once, but not as a friend. He’d been off for a week with-out contact; there were no mobile phones then, and he lived alone. I was a conscientious young manager and lived not far away, so I popped round to see if he was OK. He was surprised that his manager might be genuinely interested in his welfare, reassured me that he was fine and would be back in on Monday. We chatted briefly and I left. Twelve people lost their lives in that flat.

When I left, he gave me a farewell card he had made himself, wishing me well. It was a photo of the creation of Adam from Michelangelo’s Sistine Chapel ceiling. Laid alongside it was a picture of Concorde. It baffled me and still does. What did it mean?

After his arrest and before he was named, the papers simply said that the man arrested was a civil servant, a Scot, and a former soldier. I thought to myself: ‘I know someone who fits that description. Could it be Des?’ I decided it could be.