recruitment agency within rights to withhold salary?

Hi, if anyone can help with advice as to my current situation which I am not sure how to move on with. I am a locum worker, current assignment within the public sector. My recruitment agency refuses to release my salary to my chosen umbrella company as the agency feels the umbrella company is not compliant (previously they were releasing funds to them). The agency have given a list of umbrellas to which they say I can choose from, all of which offer a lower take home pay than my current umbrella company. What rights have i to demand payment to my current umbrella company and (b) am i better off setting up a Ltd Co instead? Any advice appreciated pls.