Contract lawyer

Hi, I tried to sell my business to a company this year, we agreed on £7000 to be paid in installments, in the contract it states that we were to hand over all stock etc upon the first installment, however this was late and as security we only handed over half, since then they have not been keeping payment. All payments should have cleared by June this year. Can i declare the contract void and walk away? they are now saying they want all of the stock but they have not agreed to pay the remaining balance, they are even threatening to sue me, and now i fear i will loose everything if i hand over the stock. Can any one tell me if i can declare the contact void?