I am considering returning to work as a temp but would rather work as self emloyed, than with an agency. As I would be using my car to travel to work, how would this affect my car insurance.
Not strictly an answer to your question, viz, but if you work directly for a firm instead of through an agency the chances are you won't be self-employed, you'll be a temporary employee. It's not something you can choose for yourself; it depends on your contract.
if you have cover for going back and forth to your place of work already,then it does not matter whether you are employed or self employed,unless you have to move around from site to site during working hours.
If you are using your car for more than one usual place of business then you will need to add class1 business use.
if you are temping and you only go one place of work per job, then you will be fine with just social and domestic and pleasure use.