Success, sort of.
In the base is a channel that the o-ring that grips the carto sits. I think that on my AGR the channel has been cut too big, so juice can flow around the o-ring thereby rendering it useless. I had an o-ring from another tank but once fitted in the channel I just couldn't fit a carto into the base, this new o-ring is just too big (though it hugs the carto well).
So...
I rebuilt the AGR with the original o-ring and a carto installed. I then ran the other o-ring over the carto and down until it is in contact with the base.
I've vaped a tankful and so far leak free.
I hope you can properly see this second o-ring in this pic.