OK, now I'm confused.
I took the car for a normal driving run and after about 20 miles the 'Check engine' light came on again. I pulled over and checked with FiatECUScan - P0401 was logged again. So I cleared the error and then drove at 3000+ revs for another 20 minutes while monitoring Particle Filter Temperature, Particle Filter Obstruction, and Regeneration Progress. All that happened was that Particle Filter Obstruction crept up and Regeneration Progress stayed at 0.00%.
So I pulled over, kept the engine running, and saved the graph and quickly checked Fault Codes, not expecting ot see any. Surprisingly P0401 was logged again but without the 'Check engine' warning light coming on. Strange. So I cleared that fault code again and drove back the way I'd come for another 20 minutes in an attempt to force a regeneration. Still no joy but P0401 logged again without the 'Check engine' light coming on.
So I cleared it again and then headed for home driving normally but also monitoring EGR Valve Opening together with Particle Filter Temperature and Particle Filter Obstruction. The images on the left below show that for the first 7 minutes or so the EGR Valve Opening value swung between 4.00% and 95.95%. But then after that it stuck at 4.00% even though driving conditions were similar to the first 7 minutes - see the the image on the right. This coincided with quite a drop in the Particle Filter Temp but I don't know if that is relevant.
When I got home I checked the fault codes again and P0401 was once again logged without the 'Check engine' light coming on. What I don't know is whether the P0401 was logged at the same time as the EGR Valve Opening value sticking at 4.00%.
Anyone recognise the symptoms I've described? Is the EGR valve fully opening and closing so often normal?
I think I'm going to have to take it in to the Fiat dealer to get it sorted. It may be the EGR valve but if it's not I've spent £130 I didn't need to. We have several long journeys coming up at the end of April and I need to be 100% certain (ok, not 100% but as much as you ever can be with a Fiat) that the car will be reliable.