Grose & Feary used rubberised cloth from the North British Rubber Co.. It is quite likely that it was bought from Handley Page Ltd as they were stockists at the time and advertised 1,000 yards of North British Rubber proofed fabric.
Very little is known about the company or their fabric. If anyone knows more please get in touch via the contact page
More on North British Rubber can be found on the following website:- NBR Wrinklies