Where in the uk is a good place to get a second hand mountain bike?

My husband and i would like to buy entry level mountain bikes. hubby is 6ft3 and i am 5ft1 so both of us need to consider frame size and would like to try before we buy. do bike shops do second hand? anything we need to be looking for when we inspect a bike? any recommendations and all advice gratefully received.
Answer
A second hand mountain bike shop? in the uk? try ebay.