Caz is correct, it is the brand/product owner and not the manufacturer who is responsible.
the legislation says that the
The responsibility for the food information shall be;
• the operator under whose name or business name the food is marketed
Basically the company who's name, brand or address is on the product is responsible for the information on the packaging.
They are responsible for ensuring the information is present, accurate and complete
Which all sounds really nice until you get to:
A Food operator who does not affect the food information shall not supply food:
“which they know or presume, on the basis of the information in their possession as professionals, to be non-compliant with the applicable food information law and requirements”.
Which means if you know its wrong then you are also liable. This clause is more for the retail of product but could equally be applied to John's dilemma.
The best approach is to put the concerns in writing (cos, if its not written down it didn't happen) and then you have advised the customer. Advising the local TSO is not a bad idea either.
If they choose to disregard the concerns at least you have advised all concerned