Thanks all. Our Contract says sellers will pay all costs of making anything legal that is currently not prior to completion. We've told all sides our Solicitor is checking into the second floor and that we will expect it to be all legal (and be able to see that it is legal) before we complete, though in fairness our Solicitor has not yet come back and said definitely that it aint. The Solicitor has asked us to provide photos of all the relevant bits - house, pool and storage so that she can check. We've done that.