Can anyone help me out.

My vertex has developed a load clunking noise whenever the brake pedal is touched whilst steering left or right.
I can feel no loss of brake performance, but the noise is very worrying.

Any ideas?

If your braking system is anything like my Aristo, it sounds like the caliper may be working its way lose. There are two 14 mm bolts just behind the disc, jack the car till wheel is off deck and tighten bolts. This way all pressure is released from calipers.

Hope this helps

Check the pads as well there is a clip on the back which sometimes can break. This alows the pad to move within the caliper.

My aristo 1993 is suffering from the same tinkering. Its due to pay a visit to the lexus main dealer - Derby, so will update you of the outcome.