Changed the spark plugs..
... and now it goes like a train!
Had my 147 for a few weeks now and never thought it was going quite as well as it should. It was due a spark plug change anyway, so I did them this morning (put NGK platinum-electrode plugs in). The old ones looked rather worn out, particularly the single-electrode ones, but they had all been firing.
Went for a quick test drive afterwards, and apart from a false warning about all the sidelight bulbs being out (cleared when I turned it off and on again) it went beautifully! It's now pulling much harder at low revs, well worth the expense. Following 2 years of trouble-free Alfa diesel ownership, I now see what all the fuss is about regarding the twin spark!