People have been reporting since about 16th April so you are a bit early too....14th April is traditionally the first day to hear a cuckoo...though how the cuckoo knows the date I don't know...:-)
You lucky devils - our cuckoo still hasn't returned, but several were satellite tagged around here last year and it's not looking good for them. There's an old saying, "The cuckoo is a funny bird, he sings his song in May, and then in June he changes tune - July he's gone away!" so I'm not giving up hope I'll hear one, till the end of May!