I am not trying to be flippant here or suggesting that none of it was true, rather, I am offering an excuse as to why I didn't ask the guide to clarify what he meant. Anyway, we moved on to the star attraction in the garden - cannabis - before I had a chance to form a coherent question, so I thought I would just look it up on the internet on my return home.
It turns out that Gelsemium Elegans had recently been in the news (August 2015) at a pre-inquest hearing into the death of Alexander Perepilichny, an exile living in the UK. There was a suggestion that this poison had been used to kill the Russian businessman, and "whistleblower" into a massive tax fraud in Russia, who died in 2012 after collapsing when out running.
Nevertheless, it was a very interesting but brief tour of the deadliest plants in Alnwick Garden, I was taken aback by the fact that many of the plants in the garden were not necessarily rare or exotic plants, but some very mundane and common varieties found in the UK. For example, Rhubarb leaves can induce a particularly horrible death if ingested.