Iain, Frogs will aim for the pond which is "ready" first, assuming there are no physical barriers. I think it's something to do with Glycolic acid. Either way, it's whatever scent algae produce, the frogs instinctively know that this the larvae's first food. During a pond rescue I watched frogs spawn in the rainfilled tracks of construction ( destruction really ) vehicles which were full of green algae despite the pond a few yards away being their historic spawning site. I've seen them spawn in househould grates where there has been algae growing on the clay top. The trick is to site the pond in full sun. A well known surveyor of Great crested newt sites used to regularly report a pond as S.O.D. Shaded out, dead. HTH.