Terry Ezra, to my knowledge, has been involved with aikido since the early sixties. I don't know who his first teacher was, but his paths crossed with those of Chiba Sensei pretty early on in the history of Aikikai of Great Britain (later the British Aikido Federation). I would guess he had quite a few influences along the way, but mainly from the Aikikai side.
I have always had a feeling when I practised with and watched Terry that he admired Tohei Sensei (though he might contradict me!), which might explain your perception of a link with Seidokan, but I would be very surprised indeed if there were any formal link.
Actually I don't think his aikido has much similarity to that of either Chiba Sensei or Kanetsuka Sensei, which is perhaps surprising, given his long-term contact with those two teachers.
When I talked to Terry Ezra about his background, he expressed an admiration for Seigo Yamaguchi Shihan. Anyone with experience under Yamaguchi Sensei would know something about Kashima Shin ryu Kenjutsu such as Chritsian Tissier does. In fact, Nick Waites, a senior instructor under Terry Ezra does reference their bokken work as coming from Kashima Shin Ryu on page 117 of his book on Aikido available in the U.K. For me, that confirms that somehow, Ezra Shihan came under the influence of Yamaguchi Shihan. I believe that to be one of Ezra Shihan's major influences.