Unite has over 3200 members. Regardless of how many didn't return a ballot all members now have the right to strike and i'll be willing to bet that most of the members will take the oppertunity to strike.
Most of the members who did not vote either did not recieve a ballot or didn't vote because they though it would be detrimental to their job to vote in favour of industrial action. The decision has been made for them and now they can be assured that they can take action.
The company must step in to try to come to some agreement asap.