Armistead was a gentle man, who worked as a railway shunter with an eventual promotion to foreman. He met Minnie at the Band of Hope, where he operated the magic lantern for lay preacher Sidney Hayes. Like Sid, who preached the evils of drink, Armistead was teetotal. He was sweet on Minnie, but she found him too serious and used to fall out with him for peculiar reasons, much as she would later do with her best friend Ena Sharples.
At a trip to Marple Lakes in summer 1919, Minnie went off on her own, leaving a flummoxed Armistead to turn to Ena for advice. Finding Ena to be the understanding, straight-talking woman that Minnie wasn't, Armistead fell for her and wrote her a number of letters offering to drop Minnie for her. Ena, who was seeing Alfred Sharples at the time, persuaded Armistead that he was being daft and that he was better off with Minnie.
Armistead proposed to Minnie during a special tea at the Glad Tidings Mission Hall and was flatly turned down because, as a railway shunter, he always smelt of oil. She only changed her mind after he became a foreman, and Minnie, weighing up the pros and cons, decided that marriage was better than growing old with her mother Amy Carlton. They were married in 1925 and had their reception at The Co-op Reception Rooms. Armistead's wedding present for his wife was a small kitten.
Moving into 15 Jubilee Terrace after getting married, the Caldwells co-existed happily enough but they remained childless as Minnie was unable to conceive. When she found out about this, Minnie ran away and Armistead had to borrow a horse and cart to find and bring her home.
Armistead was a regular at the Rovers Return and was once awarded a great big meat and potato pie for winning a face-pulling contest, accomplishing the feat by taking his teeth out.
Armistead passed away in 1935 and was buried at North Cross Cemetery. Minnie stayed faithful to Armistead after his death, and was deeply hurt when one of his letters to Ena surfaced in 1973. Ena convinced her friend that it was a moment of madness, before admitting to Emily Bishop the true extent of Armistead's infatuation with her.
- Armistead had passed away by the time Coronation Street began in 1960 but his voice was heard in Episode 1256 when Emily Bishop read the love letter from him to Ena. The picture of Armistead in the infobox is the photograph which sat on a shelf above Minnie's fireplace. Armistead's voice being heard marks a unique situation in which a character being established as deceased makes an "appearance" in the show itself.
- Some of the information on this page comes from Weatherfield Life by Daran Little and Bill Hill, although that book contains several contradictions with the main programme, notably the year that Armistead and Minnie first met, the circumstances surrounding him leaving Minnie for Ena and the location of their wedding reception.