Thanks for all the responses!
I've found it - it's inside the engine V at the front left hand side. It looks like I will be able to get to it if I remove the air supply from the front of the intake manifold. Hoping to get to it tomorrow and that the rattle will go away. I've already got a new front timing chain kit in case replacing the tensioner doesn't fix it. That of course would be a much bigger job!

