I have had an email saying the survey can be closed. What does this mean?

This means that you have had enough recipients to be able to complete the survey. You don't have to and you can wait longer to give more colleagues the chance to respond. Once the survey is closed if any of your colleagues try and complete the survey they will be notified that it is has been completed. The survey responses are then summarised and a member of the Wessex LMC staff check the summary for information that is felt inappropriate.