Thanks to my transatlantic friend Kit for this link.
She recommends a number of different blogs (and it is so wonderful that so many people out there share their experiences and techniques!) and among them was this one.
I have only just discovered it and will have to pop back later for a better read, but because we have screen printing as a topic on the C & G course I did have a quick read of this tutorial.
So, for anyone who is doing any screen printing any time soon, I think it is worth popping by Sew Sew Art for a quick read, and not least an explanation as to why soda ash is a must in dyeing!