Complex sentence: He also asserted that Gardner went on to join another of the covens, which was based in Hertfordshire, through which he received "the Second Rite of the Hereditary Craft", although stressed that this group was separate from Gardner's own Bricket Wood coven.
Split into simpler sentences: He also asserted that Gardner went on to join another of the covens, based in Hertfordshire, through which he received "the Second Rite of the Hereditary Craft". Liddell stressed that this group was separate from Gardner's own Bricket Wood coven.