It's tough but (a) other sites have been referred to OFT over not carrying private sales and I suspect that Primelocation would successfully argue that they can choose who's ads they carry.
As to asking an agent to charge a lower fee for simple referrels, I suspect that with supply so low at present they may well be tempted but I expect that they would see this as the thin end of the wedge and decline your offer. I can't believe that an agent can't add more buyers into your properties - Sorry.