If you work long-term through a temp agency, the new Agency Workers Regulations may confer on you the same rights enjoyed by employees recruited directly by the hiring company, such as holiday pay. See www.londonfreelance.org/fl/0711work.html for more on the rights as a worker you might in some cases possibly accrue if you do regular shifts.
Also, the National Minimum Wage went up on 1 October to £6.08 an hour.