Steve
Found it! Listed under Original Whitworth Pipe Thread in "Guide To World Screw Threads" by P.Sidders, mine is 20 th British Edition copyright 1969 but clearly printed a lot later.
Data:- 11 TPI, Pitch 0.09091, Depth 0.0582, Major Diameter 2.2450, Effective Diameter 2.1868, Minor Diameter 2.1286. Far as I can see its standard 11 TPI BSP / constant pitch Whitworth sizing.
Gawd knows what the tolerances and allowances are supposed to be. I guess using figures for nearest 11 TPI BSP will be good enough, or maybe those for hydraulic pipe threads would be better. My book lists both if you need (and can understand) them.
Certainly obscure, none of the general engineers handbooks I have list it, 3 Editions of Machinerys, Newnes, Caxton, and a couple of others plus info embedded in other sources. Wonder if Molesworth had it! Surprising as some of the other stuff listed is seriously more obscure, especially given the following quote from "Guide to World Screw Threads".
"Important Note: For certain intermediate sizes of pipe thread in was the practice of the engineering trade to adhere to the dimensions of the Original Whitworth Pipe threads for which there exists no official standards."
Clive