Update on this; it's taken nearly 5 weeks to resolve but seems to be fixed now.
Diagnosed as driveshafts - replaced (took 3 weeks to arrive from Japan, fair enough). Noise better but still audible.
Replaced axle bushes. Noise better, again, but still audible.
Finally, whilst recording for submission to Lexus UK for advice, the source of the remaining noise was found to be - the strut brace, as originally posted. Tightened and - for now - no noises.