For some reason I can't reply to the original question (I get asked to login but I am already logged in, and attempts to login fail) therefore I have posted my reply separately.

They seem to be confusing the law in relation to children (up to 16 years) who cannot work more than 2 hours on Sundays, with young persons (between 16 and 18) who, as far as I know, are not affected by the legislation.
http://www.employment-solicitors.co.uk/Childemploy.htm

Unless the local council has introduced a bylaw restricting the hours of work for 16 to 18 year-olds, which seems unlikely, my understanding is that they can work up to 8 hours on Sundays.

Eddie Newall