In case anyone stumbles across this with a similar problem, I will post how I resolved it.
Basically, it was imported textures from FAB (Quixel ones) which for some reason had HDR compression.
This was making a mockery of any materials they were used in.
Selecting the offending textures and setting their compression to default/normal/masks as appropriate to their usage, immediately fixed the problem.
A good question might be:
‘Why on earth were the blasted things set to HDR in the first place?’
Does HDR even make sense for a normal map, or masks?
At this point, I’m kinda like ‘Whatever…’
At least we can get back to the job of completing this project for the client; so much time wasted on these left-field diversions!