Tim Burr is an agent with MI5.
That is factually accurate but a tad misleading, at least at the start of his recorded adventures. In truth he is pressed into service without the training he could have used but sometimes (i.e., often) situations demand.
Our initial meeting of Burr comes when he is 41 years old and reasonably happy with his life. He was an employee in the civil service of Britain, working for several years at the Home Office after graduating from Cambridge. Then he was transferred to Paris to be on the staff of the Ambassador "where he would use his knowledge of foreign affairs to brief him daily with what was happening in the World. Technically he was employed as an intelligence officer. Sounded like a spy but in reality he read the local papers, checked the briefings from the various government departments and made sure the Ambassador had a clear picture of the current situation and a clear understanding of what the current policy thinking was".
All of which means that while many would consider him an operative for MI5, he likely would not have. Until events brought about by doing a "small thing' for the Ambassador based on a vague rumor and all that would change his life completely, once it stopped trying to end that life.
Burr is hardly experienced enough or trained enough to know how to properly function in the clandestine environment he would be thrust into but Burr is a very intelligent individual and he learns quickly. Over the course of the adventures we have of Burr, he will acquire that experience the hard way and become quite good at it.
His real first name is Anthony but everyone who knew him called him by his long-ago acquired nickname of Tim which we are told came from his being 'a chip off the old block, timber!'
Burr is divorced with no steady relationship though he is not against the idea. He will meet others during the adventures, some more important than others. His position and responsibilities in MI5 will undergo similar changes.