Received my lovely Mobil 1 yesterday :
6 litres for 33 Euros works out at £22.50
If a 4 litres is £39 in the UK, and 1 litre bottles are £12.99 as they were at the weekend in Halfords, buying the same volume would costs £65 (or £58.5 if you bought multiple 4 litre bottles. Why 4 litre bottles? They sell it in 5 litres in France and who has a car with a 4 litre oil capacity?
So the French version is either 1/2.6 to 1/2.9 the price of the same thing in the UK.
Just a shame my car is still on the driveway with no electrical power. I have decided to reconnect the fully-charged old battery in series with my multimeter to measure amps, and take a look at what is going on. Hopefully with some fuse-pulling I will be able to find the problem.