I drive 40 miles a day, with a mix of dual carriageways and B roads. The car's done 12.5k miles now, and I've only seen the DPF light twice... neither of these times have been on my work run - they've been when I've been driving around town with a lot of starting / stopping.
Both times the problem has been cleared with a few minutes of "Brisk" driving, legally, down a dual carriageway.
If you think about it, all the smoke that usually comes out the back of a diesel is being caught by the DPF..... so things like pulling away and putting a lot of pressure on the engine are producing soot which is being caught... and since the engine's not turning fast and for long enough to heat the exhaust sufficiently, the DPF isnt getting cleared.
I must admit, it's a major area that the VW group neglected to tell anyone, but it seems that the 170TDI is not meant for extensive urban driving
My advice is if you drive in a lot of stop / start traffic, take it for a 5 min journey down a dual carriageway at 50mph+ once a week - should keep it cleaned out.
Strangely I had the same problem as Fred (above), when the coil light came on and it went into limp mode... It needed the exhaust temperature sensor changing about a week after I bought it.... maybe a new sensor minimizes the problems....maybe not tho...