The DPF might be a total red herring, indeed I've now had the sensor replaced and they've confirmed the filter itself isn't getting clogged. It was cited as a likely suspect as it's only available on the 170 in the UK, not the 140 and, since only the 170 has exhibited this problem, attention is obviously focusing on differences twixt the two engines.
At this juncture I'm seriously considering buying a VAG-COM, at huge expense, to try and log some data when the problem occurs in the hope of finding out more.