Before I end my relationship with my beautiful C5 - the car equivalent of a MILF in my opinion Id like to contribute to the mine of good info on here.
I struggled for about 2 years with Depollution issues on my MK3 2.2 HDi. Sometimes it would go into limp mode and allow me to pull over safely. Other times she would simply cut out refusing to start until I had used either FAP or Torque and cleared the errors. The latter had a habit of hapenning on the motorway, at speed.
So 2 things I would like to suggest.
Firstly if you dont have Lexia or want to go to the expense of buying it get the full FAP app from the Amazon App store. It only costs a few quid and gives you real time info on the DPF as well as other bits and bobs. Being that it is written for Citroens and Pugs it tells you more than what the usual OBD diagnostic programs do coupled with a good OBD wifi or Bluetooth dongle. So for about £20 you can kit yourself out with a very good tool.
Secondly. My issue turned out to be the diesel fuel filter which was causing fluctuations in fuel pressure which was aborting the regen process. Replace it regularly (try not to break the nylon push connectors like i did!) If you have a DPF problem try this - a replacement filter starts from around £30 and is a 20 minute DIY job - just dont knacker the connectors or it could be a two week job like mine was!
To everyone who has provided help - Particularly @Paulh - since before I actually bought the car - I thank you! I love this car but its time to let he go. Being a fan of the fuller figure my C5 was perfect - big boot and headlights plus a comfy cabin for the long ride - my C5 was a pleasure to own. A five year old car bought in 2013 for £4k which was a bargain with 60 000miles. Overall maintenance has been around £1600 in that time for a further 60000 . Probably the best bargain of my driving life.