mikejd and big daddy, many thanks for those links, I will follow them up.
At this stage we are not looking for difficult placements, that may come at a later stage. I have a 11 yr old step daughter, 4 yr son and 2 yr daughter and would wish to control the disruption at this stage.
My understanding is that placed children are generally younger than ones own children. So allowing a couple of years for the process to work through we would only anticipating young children (2 or under) I would guess, initially.