Follow up to this, as it's always nice to put a conclusion to a fault thread.

It was indeed the MAF. I had a very helpful independent read the codes, but the only ones stored were issues I was well aware of, and wouldn't affect the running of the engine or turbo. He cleared them anyway, and it made no difference. So on a whim we swapped out the MAF for one which appeared identical (although the BOSCH part number was not quite the same). Result: Car is back to full turbo pressure available, runs better than it has in my ownership, and the full-power smoke is massively reduced. I think the economy is improved a bit too, as usually I get just over 600 miles on a tank when driving gently. This last tank I've enjoyed all the performance available to me on quite a regular basis, and I've still gotten well over 600 miles on a tank, so driven sensibly I would expect 650 or so. Which equates to about about 35mpg. Seems fairly good for a near-two-tonne, 23-year old, indirect-injection 3.0-litre engine through a non-lock-up autobox with an estate (IE less aerodynamic) body.

Total cost from the indy? £20, including the second-hand MAF. Blimmin' brilliant they are!