We have a syndicate agreement which states that "There shall be 4 syndicate members with equal shares unless all members agree otherwise". Each person has a copy of this agreement which they sign, and this agreement "shall be treated as a certificate of share".
The value of the aircraft in our group is based on the insured value and states "currently £40,000 divided into 4 equal shares".
Anyone can sell their share for whatever they want, but the fact is that the new owner would only own 25% of the aeroplane. The existing owners are given first refusal in the agreement.