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
The following article appeared in Flight Magazine on 7th August 1909:-
British Made Fabric
The North British Rubber Co., Ltd., send us one of their price lists of indiarubber proofed aeroplane and balloon fabrics, and the company claim to be the first British house that is able to offer a range of scientifically produced fabrics suitable for aeronautical purposes. Before putting their productions on the market, the company have thoroughly satisfied themselves as to their suitability by carrying out numerous chemical and physical tests. All forms of rubber articles are manufactured by them, but their specialities in aeronautical requirements are a No. 1 balloon fabric 36” wide, weighing about 9 ½ ozs. per sq. yard and Nos. 1, 2 and 3 aeroplane fabrics of varying qualities each 36” wide, the weight varying from 4 ½ to 4 ¾ ozs. per sq. yard. The No. 2 aeroplane fabric will stand a strain of 3,000 lbs. warp and 2,400 lbs. weft per yard.