if you have the same Group ID configured (or using default ID 100 for both), then I'd suggest to upgrade AP1311 to 4.0.5 release. May be the release difference from AP1451 to AP1311 is too big)
Thanks for the suggestion; I updated the firmware on both devices to 4.0.7.14 yesterday afternoon, alas no change; I hadn’t considered configuring the 2nd with all the same details; the guide suggests I just turn it on and it will join.
When adding additional AP's to an AP group the Group ID needs to match.
In my reading of the user guide, it should be a matter of powering up the additional AP and accepting a connection. I had changed the default Group ID to better reflect our environment; this means configuring the additional AP with "Dummy data" changing the Group ID.