Eventually found an exhaust garage who could do the job for £500 which included pipe tips.
They didn’t do a proper job the first time, after a couple of weeks the blowing exhaust was back, and it turned out that the way the additional pipe had been connected, caused something else to bend, opening a gap between two mated flanges
After a revisit and a different engineer, two weeks later it sounds ok.
Now back to Lexus for that engine light.
Ps what a difference to the gear change when there’s a hole in the exhaust. With a blowing exhaust I noticed that the gearbox wouldn’t change down until the accelerator pedal was pressed 70% of the way down, at which point the car would drop 2 gears and go fast.
With a blowing exhaust my Sc430 had two speeds, sluggish slow and screaming, nothing in between