David:
Yes - thanks - I see (
http://www.mediterraneangardensociety.org/experts.html).
I wonder if it is getting too much water? It is difficult to tell due to my irrigation going down a 50mm pipe straight to the roots. If that was the case, I might have expected to see water expression from the leaves or oedema on the leaves. The leaves are perhaps smallish, but in themselves they look OK for colour, turgidity, lack of PLC, etc.
I am sure the gummosis is not due to physical damage, since the exudations are all over the trunk, and the bark itself looks perfect.
Pruning in dry weather: noted. It has not been pruned at all and we haven't had rain for months now.
Where I used stones, I have put sufficient that the lowest ones nearest the trunk are shaded themselves by other stones on top, the aim being to keep the inner stones cooler. I have more Aptenia to prune; I shall use that as a blanket instead.
An aside:
Mostly, I have used Aptenia as a non-rooted mulch - I just spread the blanket of prunings out and pile it up as needed to make it deep and dark enough. But even where I have used Aptenia as a *growing* mulch, around the base of a pomelo, I have had no problems. The pomelo is growing well and has some fine fruit swelling.
Alisdair:
Capnodis infestation - I hope not! But I'll look into that. Thanks.
I will remove some of the exudations, and see if they recur.