I didn't have much exposure to Sekiya Sensei - I think I started practising at around the time he moved back to Japan. I enjoyed a day he taught in London during a brief visit a few years later, and KS invited him to lead a BAF Summer School in the early nineties, though he wasn't very active by then and didn't teach much.
My memories of Yamaguchi Sensei are much more vivid, with his visits to Oxford in the mid 1980s.
Mr Sekiya was very good. When he was in the UK, I was his otomo and often took ukemi. When I came to Japan and experienced Yamaguchi Sensei's aikido, quite a few things fell into place. YS came to Hiroshima for regular weekend seminars and I attended these for many years, in fact until he passed away. I often took ukemi for YS and, as I say, things fell into place. I also once took ukemi for Inaba Minoru and the experience was similar in many ways. But there was a sharpness, a raw edge, with Mr Inaba's aikido that was not present in that of YS.
It is unfortunate that SS and YS had a falling out, but for a few years before he died, SS trained at the Shiseikan dojo.