AO? Ah yes... that's my default two-light set. I forgot to switch off the IBL AO for my testing.

Well, this might work. It was quite easy to separate out the wall parts into five separate material zones and play with the UV settings until I got this, which I think is close enough to a dimensionally consistent pattern. Next step is to re-apply the texture and see what happens. I'm very hopeful!

Of course it was sooo much easier to see what was happening and set it up correctly using the checkerboard pattern. One of those simple things that makes one ask "why on earth didn't I think of that?"

Thanks, bb, and everyone. I'll post again with the result. but maybe not tonight, I'm nearly asleep.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)