I had the pleasure of seeing Sir Terry Pratchett at his latest book promotion in Ely Cathedral earlier this week. He is reputed to be Britain’s second most read author, and can still churn out best sellers while suffering from Alzheimer’s.

I was among an audience of 1,000 and there is no doubt that he is a much admired legend. I wonder how it must feel for him to face so many strangers out of his comfort zone, which can easily trigger a fear reaction for many sufferers of this terrible dementia.

The whole evening was led by Terry’s very able Man Friday, Rob Wilkins, who read extracts of Terry’s work to the enthralled crowd and gently invited Terry to respond, which he did magnificently. I have no idea how much of a struggle that was for him, if they worked to a standard script to aid his failing memory.

And Terry willingly posed for photos with 100 fans as he was unable to write his autograph in their copy of his latest book, Dodger, but I know this is something special which they will treasure for ever.

Terry made several references during the evening about Christianity – he supports choosing the right to die which the church is against – causing Rob to cough uncomfortably. I found it difficult to hear either of them clearly and wish they had each been given hand held microphones.

Such is his enormous appeal that the man sitting next to me had travelled from London and had to leave early to return to his home in Surrey – just to hear Terry speak for an hour.

My thoughts today are with Terry and other sufferers of Alzheimer’s disease as today is World Alzheimer’s Day, a disease which many people regard as worst than cancer.

Unbelievably, every 68 seconds, someone develops Alzheimer’s disease.