the banks valuation means prety much nothing in terms of what you should pay. it is only there to measure how much you can borrow. the banks compete with each other to offer the best valuation (the higher the valuation, the higher amount you can borrow and the more likely you will choose that bank to get your mortage).
when i got my flat earlier this year, the banks valuation was a bit under what i paid. but they may have valued it once a few years ago and then decide the current rate based on the average prices (if the flat prices on average go down by 10%, they will lower the value by 10%). so if you have a good flat in a good area, it will hld its value better than the 10% drop.