I've been looking for this topic and finally found it. Thought there might be more info. There is some talk of the cartridge on the British Militaria Forums under "Sporting Rifles and Other Old Guns".

I've also read Ken Waters and thought that his idea of using the .375 Express (now known as the .375 JDJ) would be fun and useful in the Enfield action.

Unfortunately, I'm a unable to make up my mind with all the potential projects I've got...