Mystery night-flying bird

Can anyone identify this bird for me, please? I was photographing the moon through my telescope in the South of England a couple of days ago just before 9pm and captured a silhouette of a bird passing in front. It appeared to have a long bill but not long legs, and its wingbeats were about 6 times a second. To give an idea of scale, if the wingspan was around 60cm it would have been close to 500m altitude and 1500m away (although I suspect it was smaller and therefore closer and lower). I did not hear any calls and only saw one, though of course I was observing a very small area of the sky.