False alarm!
There was a smell of exhaust gases at the front of the car,
the low-throttle ride wasn't smooth,
but that was all the symptoms.
It turns out the number-4 sparkplug was loose.
Fixed it.
Edit: the nasty looking top of the manifold-turbo bit is where oil was being blown back onto it from slight seepage at the front of the cam-cover, probably still means it isn't as tight a fit as it should be but ... meh.