diesel particle fluid level is low

Hi, I had this message on for the past 3 weeks, diesel particle fluid level is low, so finally took the car into the dealers for a forced regeneration and FAP fluid replen, hoping it would extinguish the message that pops up after starting the engine. Now I picked the car up (2.2JTD 2003 MODEL) to be told the fluid-35 pound a litre!!!, was topped up by 3 litres and a forced regeneration was carried out, however the annoying message is still there and I asked if they reset the system to ensure the message would extinguish, they told me they had. So my question is what is causing this message to come on? Is there a faulty sensor in the additive tank? Any help with this annoying message greatly appreciated.

theres quite a few posts on this if you browse through the ulysse section (search 'DPF')