I've grown these over the years in West London, but never found they survived longer than 2 years if I was lucky. They don't like heat much, and grow best in Zones 3-7, and you're zone 8, as am I, like much of the UK. They like sun, but not heat, and like moist soil, bit of a tall order for those of us in the South.

I grow Aconitum successfully in my garden in Hertfordshire, but as Bamboo says they do not like heat, mine grow in almost full shade and reasonably moist soil, although they do wilt if we have a dry spell. Mine are flowering at the moment.

Mines are suffering in the heat this year (not usually an issue here is Scotland) . I gave mine a good soak yesterday and they have picked up today. They usually look a bit scruffy around the base in my garden by the time they flower.