Horizon flyer - You might want to re-read your Air Navigation Order, specifically
Drunkenness in aircraft
242.—(1) A person must not enter any aircraft when drunk, or be drunk in any aircraft.
(2) A person must not, when acting as a member of the crew of any aircraft or being carried in any aircraft for the purpose of acting as a member of the crew, be under the influence of drink or a drug to such an extent as to impair their capacity so to act.
Hope that reassures you that the Captain has not committed any offence by having a drunk passenger on the aircraft. Back to your Cessna 150.