What's this bird in the dawn chorus?

I'm trying to identify a garden bird (Cambridgeshire) from its song, and wonder if anyone can help. I haven't seen the bird, so only  have the song to go by. It's has a two-note song. Not short, like a tit, and not 'tweety', but just two notes repeated: high,  low - high, low. Any clues would be appreciated. Thanks.