I see on Phil Needham's US Website he is starting to supply john deere 750 discs from a French Company called Forge de Niaux and reckons they're 51.7-55 rockwell C. He also seems to be able to sell them for $35 out there so they must be available cheaper for us somehow.
http://www.needhamag.com/documents/Needham_Ag_2018_Product_Guide.pdf
See page26 onwards of the guide.
Beeversales appear to be Forge de Niaux Uk importer
http://www.needhamag.com/documents/Needham_Ag_2018_Product_Guide.pdf
See page26 onwards of the guide.
Beeversales appear to be Forge de Niaux Uk importer