Once (and possibly still) almost cloak and dagger, shades of tap on the shoulder, would you like to join us here in MI5 / MI6...?, don't call us - we'll call you, the Gresham. I was a member through the 1980s, invited to join a full two years after the morning I bumped into a lovely old gentleman - he dismounting from an elephant howda, me on foot with a rod bound for a mahseer river in Nepal. He - a man from an old and hugely wealthy British family, a simply remarkable dry-fly fisher even in his eighties - just happened to be the Gresham's Chairman...
Somewhere in my library I have a privately published booklet commemorating the 100th anniversary of the Gresham, or The Grasshoppers as they were affectionately known.
Today, a call to the rodmaker Edward Barder might see you getting some info' (though not an offer of membership).