Random Anti-Pollution fault - C8 2.2 hdi

[brittleware]

On two occasions recently the engine management warning light has come on. Both were during journeys on motorways to deliver our kids to Uni/Airports:
The 1st trip after 100 miles - near the M1 end of the journey, the car lost power but would still rev above 3,000. Then after 20 minutes pottering around town (Leicester) all the power returned and the light extinguished itself.
Stayed fine until -
Trip to Luton airport at dawn. After 60 miles, again on the M1 motorway, bong, light on. This time NO power loss WHATSOEVER, no warning message, but light stayed on all the way home too.
A day later, still on, drove off, low speed local trip, light went off.
Both those trips were long and at high speed, what's going on?
(Actually it's a Fiat Ulysse - a C8/Pug 807 clone - has 86k on the odometer, Lexia reports additive system "no faults")
Do I a) defap/remap or b) fit new fap?
TIA!