GardenSlave and Collos25 are both partially right. Could be either.

This is a common problem on the HDi engines (you don't say which engine you have)

I had this as a recurring problem on my previous C5 2.2 HDi.

You do need a Citroen dealer really - it could be anything from a turbo pressure switch to the sensor which detects pressure differential between the EGR valve and the DPF. It could also be the actual EGR valve needing cleaning, or a broken plastic butterfly valve in the swirl chamber.

If your car went into 'limp home' mode when your dash lights came on, it will be OK next time you start it, but the Anti-Pollution light will remain on for the next 5 or 6 re-start/lock up cycles. Then the lights will go out and you will be lulled into a false sense of security until next time it happens, usually when you ask the car to accelerate in a higher gear.

It's a case of trial and elimination I'm afraid, the Citroen dealer will guide you through the fault codes which will only give them an indication of what component is faulty, not a definitive cure.

Sorry to sound negative. If you are lucky like GardenSlave, you will think I am scare mongering. If not you may be visiting the dealer quite a bit.