The minimum age for a Saturday job is 13 but, at 13 or 14 years of age, you can only work for a maximum of 5 hours on a Saturday. (At 15 years of age, and until the official school leaving date for 16 year olds, the maximum permitted number of hours on a Saturday is 8). You can't work before 0700 or after 1900.
If you also work during the week (e.g. with a paper round), the total number of hours you work over the whole week can't exceed 12 hours. Whenever you work, you must have a permit issued by your local authority. (This can be withdrawn if your school work seems to be suffering).
Source:
http://www.direct.gov.uk/Parents/FamilyIssuesA ndTheLaw/ChildrensRights/ChildrensRightsArticl es/fs/en?CONTENT_ID=4002945&chk=AaZA0p
Where to get a Saturday job in Norwich? Most big chain stores don't employ school-age Saturday staff. You probably need to check out independent traders. Start by asking around the stallholders on the market, then check around other smaller shops. (Although it's part of a chain, it might be worth asking at shop, just around the corner from the market, on the main shopping street, which sells remaindered books, art materials, etc. - I'm sorry I can't remember it's name but I know that they employ lots of part-time staff). Otherwise, try phoning round some of the hairdressing salons in Norwich. They might need someone to sweep up and make coffee on Saturdays.
Chris