New to agency work.
The agency offered two rates: one low rate if paid through their PAYE, second higher if paid through a third party payroll service.
It was not disclosed at the time that an additional deduction would made that in practice resulted in a lower payment than if I'd gone with the lower rate. 
I went for the higher because it appeared that after an admin fee there seemed to be small financial incentive to accept the third party payroll option.
I raised my alarm/dissatisfaction/disgust with the business practice with both the supply agency and the payroll company. Both have stonewalled my enquiry and request for reimbursement.
I feel that the only way I could apply pressure for redress of loss of income is to work for another agency so force the current agency to address my grievance.
I am currently a highly valued worker in my current placement and could very possibly be taken on permanently, hence could be a lucrative cash cow for the agency. , I am also very happy in my current place and want to be there permanently.
I feel caught. Does anyone know of an alternative way of persuading the supply agency to properly address my grievance? I am out of pocket by £150.00.