Well done Tim, you were spot on! I undid the ducting, pointed it into a bucket and started the engine. It ran quite well with barely any smoke. Ticking over, there was a steady series of oil drips blowing out of the ducting. At 1500rpm or so, those drips became a stream. So yes, the seals are clearly knackered!
So I've started removing the turbo, which means removing the entire exhaust manifold to get access to some of the bolts. Then I just need to find a replacement. Or ditch the turbo altogether and fit a non-turbo exhaust manifold. If they exist for these engines...