This is definitely not bamboo mite damage, the spots are far too small. As the bamboo mites live in colonies under their webbing tents, they produce distinctive large blotches of consolidated grazing damage representing each tent.
There are patterns of bamboo leaf damage that are not caused by pests or disease, but by leaf physiology. They may relate to temperature, water, pollutants such as ozone, or mineral deficiencies, especially manganese, or a combination of factors. Iain's pattern of damage looks similar to necrosis I noticed in April 2013 on my Borinda perlonga, but have not seen much since then. From the way the physiological damage is limited by the veins, I would guess some kind of water/temperature interaction, possibly akin to oedema blisters.
Attachment:
File comment: Borinda perlonga April 2013
perlonga necrosis.jpg [ 190.85 KiB | Viewed 3292 times ]
This is very different to the distributed feeding damage caused by wandering 2-spotted mites rather than bamboo mites, which gives a speckling on the same scale as physiological damage, but each spot is much more varied in shape, see enlargement below of the BI website pests page image, on Himalayacalamus porcatus after it had been inside the house.
Attachment:
File comment: Himalayacalamus porcatus 2009
porcatus 2 spot spider mite damage.jpg [ 107.51 KiB | Viewed 3292 times ]