I would hope things have improved in the penal system since my Dad was incarcerated, but when he was jailed for his animal rights activities, there seemed to be little desire among the authorities, either within Barlinnie prison or as part of the wider Scottish legal system, to even address – never mind remotely cater to – his vegan lifestyle.

Indeed, it would be logical to assume that, given his previous career as a police detective, the need to make his months inside one of Britain’s hardest jails acceptable in a dietary sense was probably not on top of anyone’s To Do list each morning. Possibly, given my father’s role in the hunting down and conviction of some of his fellow inmates – either directly or simply due to his professional relationship with those who did; either way, it didn’t really matter – the need to keep him alive until breakfast probably outweighed what he was going to be fed during it.

With the murderous hatred felt by the inmates towards someone quite possibly responsible for the denial of their freedom only eclipsed by that felt by the guards towards someone they regarded as being steeped in treachery – due to his having quit the force and gone over to the dark side – there can have been few places for my father to turn. At least the authorities acknowledged there was a problem: for much of his sentence, he was kept in the hospital wing for his own protection.

As regards food, the times – and attitudes – being as they were, there was little if anything in the prison larder that could fit into a vegan diet. I recall my Dad saying that he was often given cornflakes covered with tomato sauce, as this was the only thing he could eat. As a result, when I sat facing him during our visit, he looked implausibly thin, for a 6ft-plus former copper. In truth, it's his diminished bulk that remains my abiding memory of the hour we spent together. With our encounter happening over 20 years ago, I only have wisps of memory of the details, so forgive me if my recollections are anecdotal rather than verbatum.

I certainly recall him drawing my attention to an inmate sitting a couple of places to his right. As the man’s wife did her best to fill him in on the life he was missing, while their four young children squabbled or sat on the floor with colouring books, the prisoner nodded and smiled like any other father. I whispered: “What did he do?” My father winced. “You don’t say that in here, son. You never asked what someone did. You ask what they were convicted of.” I rephrased the question, with improved prison etiquette. My father’s voice dropped even lower. “He beat someone to death with a baseball bat.”

I can’t – and couldn’t – pretend that I wasn’t thrilled by this dread revelation, so my father, with the bit clearly between his teeth, went on to describe an incident a few days before. He had been chatting to another inmate and a prison warder, or "screw", had swaggered past and made some snide, derisory comment about the two of them. The inmate had watched the screw disappear round the corner and then turned to my father, before saying calmly – and with some regret: “It’s amazing the people you meet when you don’t have your sawn-off shotgun on you…”

Tomorrow: Craving life outside the wall

Twitter: @DonaldAMacInnes