Job: Store Manager (Unspecified Type) (United Kingdom)

I work as a casual helper in a charity store. Started as a vounteer but then the manager went off on maternity leave, and I was asked to step in temporarily, no experience, but I learned on the job with help from the area manager, then I was given a casual contract with pay at 5.78 an hour, however I am entitled to no holidays, and my workload is 37 per week, I must also step in when people don't turn up, I do the books, set the targets, interview volunteers, advise volunteers, work out monthly figures, do the till, do the stock, bank the money, and I open up, so I don't think that is fair pay.