Have you tried searching the Coupe club forum. If not it will give some good guidance on the hdi. There are plenty of topics on this subject, some covering the complete removal of this filter and gaining some extra bhp. Give it a go

All the blocked diesel filters i have dealt with so far have always cleaned sucessfully with iether a hot pressure washer. I just remove them and back flush ! never had a problem or one come back clogged again. Sometimes I have to cut the pipe and flush .

When PSA brought out this system in about 2002, their website stated that the filter was to be cleaned by jetwashing. UK dealers didn't wash individule filters but just sent them away to France for cleaning at a central depot & charged the cuctomer circa £250 for an exchange filter. Then of course there was the Eolys to fill, so a big bill for a 50k service.

When I bought a C5 2002 with the 2.2Hdi 3yrs old & with high miles, it was only a few weeks before I got the Unblock Diesel Filter message. The combined cat & particle filter was under the car about 2 feet from the turbo & could be easilly unbolted from the exhaust at both ends. This canister was in 2 halves held by 3 bolts around the centre so that the particle filter & cat could be separated & jet wash the particle filter. I did this on mine, about 2 hours work to save £250, not a bad return. Never had any more problems with the particle filter, but the efficiency of the engine was just dragged down by this filter. Could never get the fuel economy above about 48mpg no matter how it was driven, which was at least 10% less than my previous 13 yr old 2.5 XM. The filter being 2 feet from the manifold obviously didnt help & new designs seem to be very close to the manifold for efficiency (if you can talk about efficiency for a system that is just wasting fuel to heat up soot).