I took the plunge and ponied up for a new front bumper trim. I think I may have grabbed one of Chevronics last stock of this (there's still a rear bumper trim on eBay). Check the photo just to see what these things were like when new compared to what they look like now

(this one still has the blue protective tape on)
Turns out to be easy enough to remove the old one just by prising of. However, that's where the fun started. The new ones retaining clips don't align with the old ones. I'm guessing maybe a difference between S1/S2. Looks like I'm going to have to cut some new holes to fit this. I guess a Dremel should do the job.
The trim itself has a manufacturers date of 2010 so pretty recent from an OEM manufacturer called
http://www.silvatrim.com.br/eng/fale.asp, looks like Citroen were still supplying these up until quite recently. I wonder if Silvatrim might even have some old spare stock lying around. I've pinged them an email just to see maybe if enough people were interested we could get them to manufacturer a small run of them?
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Citroen XM (MK1.5) Prestige, 2.0i Turbo. Black