The current valuation of the property is based on the surveyor's own opinion. They will review a variety of sources and use their own knowledge to provide a valuation that they feel reflects the current market value. This valuation may not be what you expected it to be.
You are paying for the surveyor's professional opinion of the current value of the property and there is no refund for the service if this opinion differs to your or a third parties. You can send through evidence to support your own opinion and the surveyor will review this, however they are under no obligation to change their own opinion of what they feel the property's value is.