What bird is this?!

Hi there!

I hope you can help me with identifying this bird - it's driving me mad not knowing!

I live in Stevenage (Hertfordshire) and keep hearing this bird that sounds like really loud marbles clicking together. My house backs onto a small woodland area (just in case this helps identify the bird!) 

I keep looking out to see if I can see a bird I don't recognise to look up the sound on here, but never do.  Can anyone help?