It's a fairly common problem around me as i live in an area with lots of old people, most only drive the car once a fortnight and then it's to bimble over to see the grandchildren at 20mph - cue condensation, collapsed baffles and all of the soot lol. Yet to see it on an HDi though, let us know how you get on. 
Also, thanks for the reminder that i've left this guide half-complete, i think i've got some more somewhere to upload, will have to dig it out.
Also, thanks for the reminder that i've left this guide half-complete, i think i've got some more somewhere to upload, will have to dig it out.