I dont know what the answer is pendle but I've seen very little frogs to and not all of them alive, I've also seen more herons about too, I think they will soon clear up any frogs dead or alive.
I have noticed that frogs always seem to spawn in the same part of a pond each year, normaly the area that gets the most sun ie south or west facing in the shallows, maybe now that your pond is more shaded they find it less attractive.
I hope it's not a disease thats making frogs scarce this year,ill watch this thread with interest. regards Tbird+