I seem to have ended up with a lot of named Agapanthus varieties
Me too, amongst all the other things I seem to have ended up with...
I use around 50:50 multi-purpose (whatever is on special offer) and John Innes no 3 with some flint/horticultural grit thrown in if I have some around, but get them in the ground for preference (even those which 'the experts' claim to be tender).
This is 'Silver Mist' in my last garden in Suffolk - I was told to protect it in winter but never did and it flourishes.