A term or condition of a contract for the supply of goods or services is void in so far as it purports to require a party to the contract—
(a)to recognise one or more trade unions (whether or not named in the contract) for the purpose of negotiating on behalf of workers, or any class of worker, employed by him, or
(b)to negotiate or consult with, or with an official of, one or more trade unions (whether or not so named).