Prunella Scales suffering a "sort of mild Alzheimer"s"

West, who Scales affectionately calls the “skipper”, steers the boat while his wife negotiates the locks.

The programme, one of four, features narration from the couple, who gently tease one another as they bump into the side of canals and reminisce about times gone by.

West, who is already supporter of Alzheimer’s Research UK, spoke movingly as part of the programme about how 81-year-old Scales now suffers with her memory.

“Pru has this condition,” he said. “It’s sort of Alzheimer’s but it isn’t; it’s quite mild.

“She can’t remember things very well, but you don’t have to remember things on the canal.

“You can just keep your mind absolutely vacant, enjoy things as they happen and as you see them, so it’s perfect for her.”

Scales, who was nominated for a Bafta for her 1991 role as Queen Elizabeth II in A Question of Attribution, added: “Well it can be a nuisance sometimes but it doesn’t stop me being able to open a lock gate or make the skipper a cup of tea.”

As West, 79, admitted “We’re both getting on a bit now”, she said: “We’re determined to make the most of it while we still can.”

The programme will see them exploring the Rochdale Canal, the Llangollen Canal, the Canal du Nivernais in Burgundy and the Kennet and Avon canal in the West Country.

Scales said: “Do I know where we’re going or why we’re going there? Sometimes I do. Sometimes I just want to watch the countryside slide by.”

Speaking of her condition, which he described as “slight”, West added: “When you think back over the long period and you think this person that I loved and enjoyed this and that and the other with doesn’t really exist any more, it’s another person… that’s quite painful.

“But you mustn’t think like that, you must take it from day to day. You notice how much she’s able to do and how much she enjoys life.”

Scales added: “I certainly do enjoy life, and I can still perform my duties as crew.”

West, who was being filmed on the boat with his wife, continued: “Her sense of humour is still there, and things still make her laugh. We have a lot of fun.

“She’s just growing older and in a certain area more quickly that either of us would like. We’re very lucky, really.”

Watching Scales climbing around the boat and opening the locks, West said: “It’s easy to forget Pru is the age she is, when she’s so agile leaping about the locks.”

The actress admitted she has still “probably got a bit more energy than he has”, thanks to her fitness and training as a young woman.

She told the Radio Times she would like to eventually “die on the eighth curtain call”, adding: “Eight will mean the show’s been rather a success. I just hope I’m somewhere near the middle and have been reasonably good in the part.”

The couple will be joined by their two sons, Joe and Sam, later in the series as they recreate some of their treasured family memories.

West is a “celebrity supporter” of Alzheimer’s Research UK, and in 2008 backed calls for the government to increase funding to study the disease.

The programme, called Grand Canal Journeys, will be broadcast on More 4 on March 10, 2014, at 9pm.

Prunella Scales suffering a "sort of mild Alzheimer"s"