Just been to HobbyCraft, where these days it seems they are selling Green Stuff through an outfit called
GaleForce nine100g of GS for £10. Way cheaper (gram for gram) than Games Workshops 6-inch ribbon for £5.
Another advantage is that the GF9 gear comes in 2 separate containers. There is a bit of an issue with the ribbon, in that over time where the two materials meet it starts to 'cure' so that when you cut off and mix a small amount, you can feel a small hard lump in there. That's the bit that has already cured. With the ribbon I always cut off and discard the bit at the join before mixing the remainder (if it's brand new it's not so much of a problem)
They also sell GF9's 'Grey Stuff' - similar to 'Green Stuff' - not sure what it's like, so I bought some and will have a little play around with it to see in what way it's different to the other things I've used. Watch this space, as they say!