Garage did read codes - Which is what pointed them at the EGR and then Turbo lines of enquiry. As you say, intermittent faults are a real PIA, which is why I thought I'd ask around here on the off chance.

So, I have a 2010 Disco 4, 3.0 TDV6. A while back I was cruising on the M40 at 75 mph with cruise on, and the car started to struggle to maintain a steady speed. It was as if the car was having momentarily breathing issues. Eventually the "Reduced Performance" warning and red triangle came on. I...

I’ve got grabber AT on mine and road noise is horrific. Had wheel bearings, prop bearings etc checked, all good. Tyre change next.