Thanks for the tip. Unfortunately I'm about 250kms from the nearest diagnostic test facility. On checking after I posted the query I found the wastegate stuck. I freed it off and tested vehicle under load. No real improvement. Once up to speed, at about 3000rpm, I can maintain a speed of about 110km/hr. Under load, uphill, down to second gear at times. Fuel filter changed (hopefully) about 6000km ago at service. I'll replace anyway.
I'll try the "reset" route. Could it not possibly be the fly-by-wire throttle mechanism - dusty/dirty?