I'd say gazumping is the least of your worries.
If you haven't sign a contract than you are only sale agreed, so yes someone else could offer more, but there is very very very few people buying so i wouldn't worry.
You can make sure that the sale agreed sign goes up and its change to sale agreed on any advertising. That means that no one else can come to view the property. If you have mortgage approval in place and your not waiting to sell anyother property, then i'm sure the EA will have no problem with going sale agreed.