This subject has been covered many times in many places. People have, and will continue to fly illegally, as long as they continue to get away with it. I know a few people who do this but if they are being paid cash in hand, it's difficult to prove.
If a crash were to happen, and a survivor admitted they were paying the PPL pilot, then you might have a case. Especially as any decent insurance company would refuse to pay out, and rightly so.
Corporate flying was always a grey area with one guy I heard about flying a A109 on a PPL with 200hours TT. He only picked customers up for his boss. He wasn't paid as a pilot. Not breaking the law, but for those of us who bothered to get a CPL/ATPL/IR etc, it is annoying.
He did go on to get a CPL though!
I don't know how you would go about grassing these people up and getting satisfaction. Ramp checks? A CAA hotline? Any thoughts?