I'm pretty sure that the old rateable values were done in the 19th century. My Mum and Dad's house, built around 1880 with 8 bedrooms, 3 acres of land had a lower rateable value than my bungalow, which was built in the 1860's but extended in 1969 when it had its ratings adjusted. Rateable values were thus set in stone for unchanged older properties. A house built in the 1970's would always have a higher rateable value than for example a similar 1940's built house (in the same area) I think the bandings were introduced in the 80's but nobody ever came down here to value the property, or my parents house for that matter. Once the bandings had been decided my parents house came out just one band higher than mine. just ridiculous
EDIT, just checked the latest banding on my parents old house and it went up to Band H in 1993
Pete