Drew Dingley and Hannah Deacon say cannabis oil can save the life of their son Alfie, who suffers severe seizuresJACK TAYLOR/GETTY IMAGES

An army of 200 people descended on central London at the weekend to hear speaker after speaker extol the virtues of cannabis. There was not, however, a stoner in sight: the audience was almost exclusively men in suits or chinos.

Welcome to the Cannabis Invest conference 2018. Woodstock, it was not. The most exotic items consumed were bottles of mineral water, and the audience of institutional investors had travelled to the five-star Mayfair hotel to hear about the financial opportunities in the burgeoning international cannabis market.

In nine American states marijuana can be bought and sold legally for recreational use. In a further 13 states the drug has been decriminalised and in all bar a handful it is legal for medical use. Later this year…