Well Moto support have come back and it appears (yet to be fully tested), that Fast Roaming is at the root of our problems. Apprently this should be turned off UNLESS you are also using 802.1x. We have turned off fast roaming and the results are positive at this stage.
So from the manual ..
Define the Fast Roaming configuration used only with 802.1x EAP-WPA/WPA2 authentication.
NOTE: Fast Roaming is available only when the authentication is EAP or EAP-PSK and the selected encryption is either WPA/WPA2-TKIP or WPA-CCMP.
Using 802.11i can speed up the roaming process from one access point to another. Instead of doing a complete 802.1x authentication each time a client roams between access points, 802.11i allows a client to re-use previous PMK authentication credentials and perform a four-way handshake. This speeds up the roaming process. In addition to reusing PMKs on previously visited access points, Opportunistic Key Caching allows multiple access points to share PMKs amongst themselves. This allows a client to roam to an access point it has not previously visited and reuse a PMK from another access point to skip 802.1x authentication.