Thanks Nan. Yeah tansy is invasive alright. I'm planning to pull all those roots out but would also use a cardboard barrier under the bed if I think there are any left. Their density in the coals is not as bad as in regular soil or even gravel.
Just reading about biochar last night, and it seems it is recommended in very small amounts - 2% is 'enough'. So although it's old stuff, maybe should shovel some out of there and mix with compost, let it sit.
Or maybe I should go back to the original plan - ditch beyond the garden.