Well I decided to keep the soaking water when I drained the beans to cook them, boiled some scrap fabric in the coloured water, let it cool down added vinegar - curiously the colour turned red at that point!! - and let it soak for another two days.
When I rinsed and washed it, it turned out a lovely lavender colour. Instant win!
All other fabric printing and natural dye action to be found here.