Nice to meet Peter this afternoon (even if it was grey and dank here 
)Lexia met C5 and reported a slew of faults related to the MAF sensor. Ah, thinks I, I've got one off the Xantia I can stick on for a test, but no.....it's a different diameter frustratingly! Then just as we were about to close the bonnet I noticed the wire to the MAF looking a bit odd just where it is under the air pipe - a little investigation reveals the wire is rubbed through the insulation. A ha! Freed it off a bit (it's in a right awkward spot right alongside the fuse box), then taped up the exposed wire, and cable tied it back out of the way. That sort of failure ties in with the faults recorded, and sporadic nature of the fault, so I'm cautiously optimistic...
wait till it's been driven a bit and see whether there's any improvement 