Tam Dalyell, who has died aged 84, was a one-off. He was one of the first MPs I encountered as a young journalist. He was already a legend. He had left his mark on history, the West Lothian question. Typically, it was not his name that he gave to his question, but that of his constituency. He was an opponent of devolution to Scotland the first time round. In 1977 he asked why an MP for West Lothian should be allowed to vote on matters affecting England, but not on matters concerning his own constituency – which would be the responsibility of a new Scottish parliament.

I met the legend four years after Scottish devolution was defeated in the 1979 referendum, and after Labour was defeated in the 1979 election. Dalyell had taken up the campaign against Margaret Thatcher over the sinking of the Belgrano, the Argentinian warship, in the Falklands conflict in 1982.

With his characteristic enthusiasm he accused Mrs Thatcher of ordering the sinking – of a ship sailing away from the Falklands – to put paid to an American-brokered peace plan. As a reporter for the New Statesman, my first job in political journalism, with the 1983 election coming up, this was heady stuff.

He pursued his campaign tenaciously. It had no effect on the election, which Labour lost disastrously, but he carried on. At one point, when I met him in the House of Commons to discuss it, he marched me to the Table Office to ask the clerk there to explain why he, Dalyell, was not allowed to ask any more parliamentary questions about the Belgrano. The clerk politely explained that he had asked hundreds and that they were “repetitive” and therefore “out of order”.

As the indignant Dalyell marched me back to his office, we almost literally bumped into Mrs Thatcher herself. She only came up to my chest. She and he acknowledged each other briefly as they passed.

By then, I was already wondering whether Dalyell’s campaign was right. Years later, it emerged that it wasn’t. The commander of the Belgrano gave interviews saying that his orders were to destroy British ships and that his zigzagging course had been intended to avoid detection.

Dalyell went on to pursue other even more outlandish Falklands-related conspiracy theories, including the idea that Hilda Murrell had been murdered in 1984 because of the military connections of her relatives, or because of her campaigns against nuclear weapons. The New Statesman took up the case, but I was unconvinced.

But others of Dalyell’s campaigns were soundly based. He brought the same unflagging energy to campaigning for the rights of the Chagos Islanders, shamefully moved from their home on the Indian Ocean islands to make way for the US military base at Diego Garcia. Unfortunately, his consistent anti-Americanism weakened his arguments.

He was, however, unfailingly courteous. He had an unusual background for a Labour MP. Eton, Guards and Scottish laird of The Binns, Linlithgow, in the aforesaid constituency. He would write letters agreeing or disagreeing with me – mostly disagreeing by the time we got to the Blair years – but always in good spirits and with a love of life.