richatom
All I can suggest is, as I have said before and Bravo has implied, is that you should not arrange the aircraft hire. Keep as much distance as possible from that side of it, let the customer arrange a price with the owner, and pay directly. You might not get immediate problems but if you are involve in an incident, maybe as small as a passenger falling, then you could end up in court and find you have hefty liability. If the passengers claim you arranged the whole thing, then you could suffer a fine and risk your licence as well. The DGAC might say one thing, but their enforcement branch and a court might disagree - that is where the law is tested!
Remember, it isn't just what is technically legal that matters, but what you can justify to a court.