Expenses through agency

I work through an agency freelance. I am struggling to get expenses paid i.e. mileage and parking. To be honest I think I was duped a little by being told I was on one local project for a year. Then ending up on four different ones. I agreed with the agency that I wouldn’t be paid for travel to and from work. At the moment I am owed around £500 from April. I am looking at money claim but was wondering how interest is calculated for each month? I haven’t claimed for travel to and from jobs just from one site to the next onwards etc?


The setup is my line manager is supposed to authorize the expenses then send to the agency. This is proving difficult for some unknown reason?