Just thought I'd report back.
I've made a small hexagonal array of 7 transducers spaced 2Lambda apart (1 in the middle and 6 around) and it narrows the beam width from 70 degrees to about 30 degrees. Curiously there is a dip on the bore-sight.
Not that a small dip on the bore-sight really matters for my application but my thinking at the moment is that there is some acoustic coupling going on through the board that is causing interference and hence the dip.