I have, as well as repotting the tomato plants, been clearing behind the shed where a lot of the blackberries grow. It is completely overrun with nettles, those plants that stick to you, and the horrible purple weeds (I really should learn their names)
It is this last fact which is central to this blog post. Thanks to @Dracunculus2010 for the tip which inspired this.
Apparently, if you soak nettles for a week or so in water (hopefully a water butt, but I don't have one of those so an old bin is sufficing) it will "make great plant food and it's free!"
I can see myself having a challenge when it comes to decanting the liquid, but I will keep you all informed with how it goes and how I solve those challenges.