A big range of different experiences (all v.worthwhile to hear), so thanks to all.
We have a travel-system infant seat that can be lifted in and out of the car and clipped to the accompanying buggy. Nowhere on its guide does it say 'don't use for more than X period' in any one day, but common sense does dictate that v.small babies are delicate and it does make sense that a 2-week old child is not tough even the way a 2-month old is.
Just to clarify - this seat is a rear-facing, reclined 0-9months child seat without a base - not an upright one. I recall my parents lifting my little brother and sister in and out of the car in an unrestrained carrycot - the car had no airbags, head restraints or rear seatbelts, so I know that we're not comparing like with like!
I understand also that EU states have similar law governing these things and we can expect French cops to sling us in le-nick for travelling with an unrestrained child, so it's about travelling safely for all concerned rather than getting from Spain to Cherbourg at any risk.
I was thinking also of putting them on a cheapie flight to Charles De Gaule or Orly - or on a train to Paris or near it - which would be a compromise.
My problem is - perhaps naively - not that the child might cry the whole way or demand constant stops (an entirely realistic prospect in fairness), but rather the idea that the rear-facing seat props a newborn up to the extent that it causes a compression of the chest and restricts breathing, perhaps too much for a v.young child after a while, even if the child is healthy.
The teller of my horror story went as far as to suggest the death of the child, and I don't have to explain to any parent the sort of doubt that this creates about a course of action that was previously thought not problematic.