Do I need liability insurance if I'm not running a business?

Basically I'm a trainee motor mechanic currently at college and looking for an apprenticeship but I already know how to do most jobs on cars already. I would like to advertise my services in the local paper to do simple jobs eg. Brakes, oil and filter change, spark plugs etc. Here's my question - if I do not open a buisness, have a business name or premises do I still require liability insurance? Or if I do is there any legal form which I can get "customers" to sign to say that I am not responsible for any incidents which occur after I have completed the job?