Flashing (with diesel particle filter)
Diesel particle filters must be cleaned.
Continue driving and as soon as the road and traffic conditions permit it, increase speed to more than 25 mph (40 km/h), at which point diesel particle filter cleaning will start. The control indicator goes off as soon as cleaning is complete. We recommend leaving the ignition switched on during the cleaning.
Further information – see page 197.
And page 197 says
Diesel particle filter
The diesel particle system filter polluting soot particles out of the exhaust.
The system contains a self-cleaning function at certain intervals. The filter is cleaned by burning the soot particles at high temperature, This procedure runs automatically under certain vehicle conditions and can take up to 25 minutes, during which time fuel consumption may increase. The smell and the noise that occur are normal.
The system cannot automatically clean itself under certain vehicle conditions such as driving short distances.
If the control indicator (glow plug light that is)
flashes, you should continue driving and as soon as the road and traffic situation permits it, increase speed to more than 25 mph (40 km/h) and diesel particle filter cleaning will start. Cleaning is quicker at faster speeds and under load. The engine speed should not drop below 2000 rpm. The control indicator goes off as soon as cleaning is complete.
We recommend that you do not turn the ignition off during cleaning.