Buddypress - Join Group button acting nuts

/groups --> lists groups correctly.

...and I can successfully join a group by clicking the "Join Group" link listed under each group.

But click into an individual group, and hit the "Join Group" button and I get the "Wordpress Failure Notice" page "Are you sure you want to do this? Please try again."

The curious thing is that both the above links are the same (e.g. http://narrativetherapy.tv/groups/narrative-therapy-theory/join?_wpnonce=b81adb998a) but one works, and one doesn't.

The other curious thing is that if I paste the link into the address bar, it fails also. So it seems that the "Join Group" links on the /groups page do something clever that the "Join Group" buttons on the individual group pages don't.

Have I discovered an obscure BP bug and if so do I get a cuddly toy prize?

Any suggestions to fix the "Join Now" buttons much appreciated.

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BP v1.9.2
Membership v3.4.6.1