The administrative trustees may sell any of the chattels from time to time vested in the custodian trustee upon the trusts hereof and where any such chattels are sold the net proceeds of sale may either be dealt with as capital money in accordance with clause 6a hereof or may be applied as if they were income of the Chequers Trust Fund or may be invested by the administrative trustees in the purchase of other chattels of the same or any other nature which when purchased shall be held by the custodian trustee upon and subject to the same trusts and powers as the chattels sold provided that the power to sell chattels under this clause shall not be exercised except in pursuance of a decision of the trustees in which the chairman of the trustees concurs.