I have an answer, one I am not sure I like, but understand.
My car has completed 120,000 miles with the majority done long distance at high speeds. So, I was surprised when the DPF warning light came on as there should really no soot build up as such. What there is generated should have been burnt off. I took my car to Clive Atthowe in Norwich, who I have known a long time and totally trust, who specialises in engine tuning and has the franchise for DPF cleaning. He analysed the car and exhaust system and confirmed there is no or very little soot build up in the exhaust. The Citroen computer calculates based on mileage and use when it decides the Diesel Particulate Filter has come to the end of its life, irrespective of the condition of the filter. So, the only way I can clear the fault and avoid 'Limp mode', is to have the system cleaned and reset to zero. I appreciate it is a filter and filters have an end point. But in my case the computer says 'it has' when it definitely has not.
Anyway, a clean is definitely cheaper option than a new filter.