U can get details from the bank say u want to mortgage your property to give u list of licensed land valuvation officers in your area.
Dont go to Grama Sevaka, they are not people whom u can NOT trust .
Here is the Department of Valuation details
Valuation House’
748,Maradana Road
Colombo 10.
Sri Lanka
Telephone:+94-011-2694381
Fax: +94-011-2694382