Yes Barrhead (whom I assume are agents) are correct.
Many tour operators offer a “low deposit” scheme where you only pay a portion (usually half) of their normal deposit when you book early. It is (or should be) made clear when booking and in the T&Cs that the full deposit amount will be forfeit in the event of cancellation. I’ve just made a holiday booking myself for 2014 and paid a low deposit. The situation regarding the balance of the deposit being payable in the event of cancellation was made perfectly clear to me.