Complex sentence: It is difficult to conclude that Weston was not still affiliated to the SAAC, but this was not to last, and it is not known if Weston joined any other fighting group until the middle of the following year, 1916.
Split into simpler sentences: It is difficult not to conclude that Weston was still a member of the SAAC, but this was not to last. Moreover, Weston does not appear to have joined joined another fighting group until the middle of the following year, 1916.