Where you start doesn't matter: you can pick the first single in 2 ways,
the the couples can be seated in 8*6*4*2 ways and the remaining single person can then be placed in any one of the 3 places between the couples.
2*8*6*4*2*3/9! should be the probability = 1/1260
I think this is correct, but I am not absolutely certain. If you get another & better answer, I would like to know about it. Thank you.