i second what several people have said:
– coal ash is toxic for the garden
– wood ash is good either in small quantities in your compost heap, or directly around fruit trees and garlic (my garlic had an instant growth spurt after i put it on). it provides a potassium boost.
– charcoal finings DON’T add nutrients (it’s almost 100% carbon) but DO provide nooks and crannies (surface area) for soil bateria and fungi, and water regulation in soil, thus helping long-term soil health. some permaculture types swear by ‘charging’ the finings before application, by using urine or nettle/compost tea, or putting it in the compost heap instead of straight on the garden. i haven’t had time to do this yet so i’ve just been scattering it on top of my mulch.
– you could break up ordinary charcoal to make finings, but do see if you can find some charcoal made in UK woods, as mass-produced supermarket stuff is pretty destructive to environments overseas, whereas charcoal-making is making a comeback in the UK as part of reviving woodlands here (google coppice / woodland management for your area, or a local organic/permaculture group will know where to find it), or DIY-it as someone said
