"It's Chris," my mother said on the phone. "He's on the way to Addenbrooke's in an ambulance. It looks like a stroke." She is an ex A&E nurse. She was right.

Three anxious weeks later, Chris was at home recovering from paralysis on his left side. He was lucky; his speech was not affected, his memory seemed to be intact, but writing was a challenge and walking, even as far as the front door, an exhausting effort. He was at the stage of recovery where he needed a rest after getting dressed in the morning, and another after breakfast. It was tough. I was back at work, commuting.

Casting about for ways to cheer him up, I hit on the idea of writing little notes on the morning train and posting them at King's Cross. They would at least encourage him to walk as far as the doormat and keep him entertained for a few minutes. If he remembered them when I came home, his memory was working. If he laughed at them, his sense of humour was working too.

I started off describing people on the train, the day ahead, and so on, but that source ran out after about three days, so in desperation I began writing in different characters’ voices. Several eccentric ladies made an appearance: there was a 19th century artist's model, Fifi Laforchette, who wrote in a French accent and risked her virtue daily among the disreputable artists of Paris; there was Arabella Mountfitchett, an aristocratic 18th century lady who had been washed up on a remote Polynesian island and supposedly sent her letters by bottle; and there was Sister Boniface, a member of an impoverished religious community in the Fens.

I remember writing some of the letters standing up, crushed on all sides, or squashed among people's cases on a luggage rack. It was huge fun writing them. They clearly offered an escape for us both.

The letters developed a life of their own. Sometimes I would find a reply in Chris's wobbly handwriting. They were hard to read, but they were hilarious. He strongly advised Fifi to keep her boots on at all times during her life model sittings. In the persona of a country parson, he carried out dangerous experiments in steam power, setting fire to several hay ricks. Oddly, we rarely spoke directly about the letters, except to say that we'd received them: "I had a terrific letter in the post today." "Really? Me too!"

Lady Arabella and Fifi were entertaining, but it was Sister B whose letters took off. It was easy to find comedy in the clash between harsh 21st century reality and the gently indomitable innocence of the sisters. Their convent is broke and the authorities want to close it, but they decide to resist. Chris and I were resisting being shut down too, in a way.

Eventually, I stopped commuting and found local work. Chris' recovery continued and we both forgot about the letters until we decided to move house. The usual huge clear-up began and I found them in a pile. A big pile! They added up to 30,000 words – nearly a novel, I thought. (I was wrong about that!)

I'm glad to say the letters still make Chris giggle. I continue to write at the crack of dawn, but these days I get to sit on a chair, instead of a luggage rack.