Sounds like you (sorry, your plant) has spider mites for sure. At least from you description and appearance of the plant I would bet on it.

These can be very difficult to get rid of, but whatever method you use, persistence seems to be necessary. Google 'spider mite control' and you'll find lots of recommendations. I can't say for sure which method of control is more likely to succeed. Sometimes you just have to pick one, try it out and move on to something else if it doesn't work.

It's often said these are very contagious, but that hasn't been my experience. I had a bad outbreak on a couple of plants this winter, but it never spread to plants just inches away. However, if you can isolate the other not-infected plants, it's probably a good idea.

Kevin

Thanks very much indeed for the advice, Kevin. I am much obliged.
I have found a product reputed to be effective against spider mites but pesticide free and safe even for plants where kids are in the environment. Now if I could just find where these things came from... (the mites, not the kids)
So a couple of further questions arise.
Should I slice off the infested yellow leaves to reduce the amount of treatment the plant needs, or should I just let them wither and fall as I would with a normally ageing, but healthy leaf?
And another question came to mind when I was pondering that question: what is best to use for sterilising blades and such when dealing with infestations and similar problems? Flame? Alcohol? Chlorine bleach?
Or are they all as effective?
Grateful for any advice.

Yes I would cut off the leaves. As far as sterilizing goes, I've never heard of using bleach, but it probably would work. I think the flame or alcohol would do the trick just fine.

Critters on plants are funny (not ha-ha funny, just strange) I always find it interesting that some plant may become infected and other plants nearby aren't. I've noticed the same thing with scale, aphids and mealies. It's understandable if a plant is weak or sick, but when one perfectly healthy plant gets something and other nearby plants don't, I always wonder why.

Kevin

Kevin, thanks again for the help.
Leaves are cut and binned. And spray just arrived, so tomorrow I will give it a good drenching.
In the past I have also noticed the situation which you describe -- especially with mealie bugs: infesting an otherwise healthy plant, but studiously ignoring its neighbour.
But I am keeping all my others well away until I have the phal back in good health.
Bests, BR

I have spider mites affecting some of my Alocasias downstairs, but they haven't spread to my orchids, or the banana-family plants I have down there. They also haven't touched my Monstera plant, either. I'm thinking that they prefer to attack plants with a leaf surface that's easy for them to pierce. The plants with the thicker, more waxy cuticle seem to be left alone by mites and mealies, in my house.

Well, the treatment does not seem to have done much so far, and I will have to keep on keeping on. Looks like I am losing another leaf to the mites. Am starting to wish it was a yucca, zzyzzyx...!
But Kevin did warn me this might take some time.