Thanks @PolynomialDivision - that was quite helpful in that it prompted me to come up with a way to easily test the neighbour reports. First set up a wpa_supplicant hanging off the AP with the neighbour reports active. Then use the wpa_cli command "neighbor_rep_request" to trigger a request. The mac addresses have been modified.
It appears to be working.
>neighbor_rep_request
OK
<3>RRM-NEIGHBOR-REP-RECEIVED bssid=14:xx:xx:xx:xx:c9 info=0x9ef op_class=81 chan=11 phy_type=7
<3>RRM-NEIGHBOR-REP-RECEIVED bssid=14:xx:xx:xx:xx:c8 info=0x19ff op_class=122 chan=108 phy_type=9
<3>RRM-NEIGHBOR-REP-RECEIVED bssid=14:xx:xx:xx:xx:e4 info=0x19ff op_class=122 chan=132 phy_type=9
<3>RRM-NEIGHBOR-REP-RECEIVED bssid=14:xx:xx:xx:xx:e5 info=0x9ef op_class=81 chan=6 phy_type=7