Yes, the first lot of tadpoles should have left the water by now, but sometimes the conditions aren't right for all tadpoles to metamorphose - sometimes it's overcrowding, or not enough nutrition, or maybe even the wrong temperature? Not sure about that one. Sometimes tadpoles go into a sort of stasis and remain as tadpoles over winter, so they will need a pool that is deep enough not to freeze solid, with the right sort of mud at the bottom, plants and rocks etc.
If you're not sure what you're doing it might be an idea to get some advice from your local Wildlife Trust - they should know of a local herpetologist who can advise you. Way back when we were kids there were so many frogs that the loss of a few in a tank on a sunny classroom windowsill wasn't a tragedy, except for the individuals concerned. Nowadays as individuals we have a responsibility not to cause more damage than our race has already done.
Just a thought - where did these tadpoles come from? They are British Native frogs aren't they? I mean you wouldn't go releasing imported frogs into the wild would you?
