Ouch £28 for a fuel filter... I guess its tolerable but it does make you wonder how much we're getting ripped off for parts on more modern cars which seem to be getting ludicrously expensive and must surely be many times the manufacturing cost...
Regarding the light - I think it depends on the ECU in question... I always thought permanent faults would remain in memory until reset with a tool like a Lexia but I came across a passage in the technical documents for one of the ECU's in the Xantia, I think the Bosch MP7.0 for the V6 engine which said something along the lines of "a fault that does not re-occur in 40 starts of the engine will be automatically removed from memory", in other words a permanent fault such as a faulty oxygen sensor which is then replaced will clear itself after 40 stop/start cycles of the engine...
It's probably still a good idea to put it on the Lexia anyway in case there are other related or unrelated faults.
It's been snowing all night here, we went visiting tonight and in the time we were out over 30mm of snow fell on the car and the unsalted roads where we were visiting... it was the first time that I've actually needed to use the snow mode on the gearbox, which worked very well - although even in snow mode I needed to be gentle on the throttle to climb a steepish hill to get back out onto a main salted road..a bit too much welly and I could feel the driving wheels taking turns letting go, likewise a fairly gentle jab of the brakes even at 20mph was triggering the ABS, so not much traction...gentle throttle and brakes and it was perfectly fine though.
C4 threw a strop this afternoon - SWMBO was overtaking a tractor and it dropped into limp mode. It seems to run OK just now (driven gently), but the EML is lit, and it comes up with depollution fault soon after starting. It also chucked up a handbrake fault message for good measure but that may be just because it was in a strop! Nothing obvious out of place under the bonnet, and there were no odd noises or other warning signs. Hopefully get it on a Lexia at the weekend -but are there any common ones on this engine (yeah, lots I hear you say, and most end in Turbo and cost £££££ 
)
How on earth do you get it out???
Well, 10 minutes to find something to remove the mounting screw whose head was completely rusted to nothing with 14 years of road salt, THEN 5 minutes to swap the filter... 
The weather report says otherwise, and its well and truly blizzarding up this way...
Looks like the original filter going by the casting date on it. Been out for a run in it, and despite trying hard, I couldn't provoke limp mode, so hopeful that is the right route.