Good idea, but what seems sensible to you or me, it doesn't necessarily follow that the council, health & safety, the Environment Agency, or other bodies would see it the same way. You must know that councils up and down the land tear buildings down if they haven't received the proper planning permission. I don't know whether such bodies have become more flexible in recent years as I, thank God, have never had to deal with them. But, let's face it, such bodies do attract a certain number of pettifogging officials who relish the chance to be bloody-minded.
From a practical POV the land area available should provide plenty of space for your suggestion (new unit, then fill in old hole). The land area outwith the PSTP is currently grassed over.
MM