Solutions gathered by Alan after RI lecture:

 

Polo total	Number of distinct responses
9	2
10	6
11	10
12	13
13	10
14	5
15	12
16	8
17	3
18	3

In fact, Cecilia, Anna and David have emailed us to say that they think the above table isn't quite right.

They suggest the following:

Polo total	Number of distinct responses
9	3
10	6
11	10
12	13
13	10
14	10
15	13
16	10
17	6
18	3

They say:

It is possible that we lack some solutions, but what is absolutely certain is that the list must be symmetric since for every solution in which each row and column add N we can find a solution in which each row and column add 27-N, with the following procedure:
a b c           9-a 9-b 9-c
d   e           9-d     9-e
f g h           9-f 9-g 9-h

Thank you!