Hi thanks for your reply 😀
I don't know where William Butler worked before or after Whitefriars, as other than his Whitefriars period, I can find nothing about him, but I've seen images of his signature on the internet (but I can't find them at the moment 😂) and it appears to look the same!
Yes, Baxter designed the shoulder Vase in the 1960s which was neater, more balanced and textured, but to me, it loosely or slightly resembles the shoulder vase? http://www.whitefriars.com/contents.php?ID=150
If the signature (and the glass) is right, he thought enough of this piece to sign it, maybe it's an experimental piece?
Could the blue glass be blue cloudy glass please? http://www.museumoflondonprints.com/image/143274/james-powell-and-sons-whitefriars-ltd-vase-in-blue-cloudy-glass-20th-century
Thanks for your help