So, on the same car, I've just blown the turbo, and (once I let it all get cold) it ran again OK, and after one burst of oily smoke, the engine runs clean. I'm not going to swap the intercooler again; I reckon unless the super- or turbo-charger has let go (spreading bits everywhere) the airflow will clean it out OK.
Here where I get to eat my words.
I changed the turbo, and gingerly set out to test drive the car. As soon as it reached 1500 rpm, it started to suck oil out of the intercooler, and started to run away. So I jumped on the brakes, and dumped the clutch. (Nasty!!!)
Three or four cycles of that, and I could get to 2000 revs, so could get movement.
Drove to a nearby hill, and drove up there in the gear that allowed the throttle to be wide open, but the engine around 2000-2500 rpm, and it seemed like the intercooler was dumped out of the exhaust! (Apologies to the 1950's bus going up the hill behind me; I'm sure everyone thought it was him, but "it was me!" ) )
Three more runs in total, increasing the revs and throttle up the hill, and I finally had full throttle and almost full revs, with out run-away or excessive smoke.
So a delicate and risky procedure, and lots of nasty environmental effects, so I can now appreciate that there have to be better ways of cleaning out the intercooler. (Still favour a good drain plug...
Apologies for my comment earlier; my method might work, but is not appropriate for the modern servicing department.