Baddabingy i use a company called valex/megaw and hogg national valuers (recently purchased by CB Richard Ellis). I compare them with previous valuations done by the same company.
Sometimes they come in over what i expect and someimes under (probably 5-6% max either way).
Im not really a pro in doing valuations, probably would be better for you to contact a professional valuer to see what methods they use.
If i was just purchasing a place for myself to live in probably the "past sales in the street or vicinity" would be something i look at.
