As I wrote above they did not have a long history of correspondence (previous correspondence was mainly with a Gordon Mosley).

The letter written by Russell was preceded by a letter from Mosley (maybe trying to bait BR) on "the root differences between us" in December 1961 to which BR replied with two letters before Mosley tried to invite BR for a private lunch which prompted the letter of note response. I think this makes perfect sense, he initially engaged intellectually, but when invited to associate privately he strongly refuses.