Personally I wouldn't be using the council rates as anything more than a very loose guide. They're rarely tied to what the market see's as value.
In my opinion the most accurate way is to get as many comparable sales together as you can. If there's a house on the property already, find out what others in the same condition and land size have sold for; then compare these sales to other packages of land of the same size but the difference is the quality of the house. After you've done this enough you'll have a thorough understanding of what the market land value is.