Cam Belt Noise

A cyclical sound started emanating from my ZS today which is hard to describe but sounds a bit like a clockwork knocking sound, very audible from the cabin, and, bonnet up, seems to be a noisy cam belt. I've had a small power steering leak over the past few weeks that I had hoped I had cured by fitting new jubilee clips, but today in that part of the engine it looked very grimy again. Could the power steering leak effect the running of the cam belt, or could the grime be from a failing cam belt? If the cam belt is failing how long do they usually last for? I'm away for Christmas, so can't check through its service sheets to check when it was last replaced. Of course, the sound could be something else entirely! It did have its MoT earlier in the day (a slight fail!) which I'm sure is unconnected!

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