All of a sudden today, potentially after going over a big bump in the road (but maybe not due to this), the boot is rattling on my Superb. 
 

It sounds metallic to me. Only happens going over rough road / bumps. 
 

Anyone experienced similar before?

Could it be your rear suspension? Mine sometimes gets a bit noisier going over large bumps depending on the weather (and it's been hot round here for the last few days).

I’m not sure. I did wonder if perhaps I’d snapped a spring, so I’ll check that out tomorrow and see if I can see anything. 
 

I strapped the jack down a lot more, but sadly it wasn’t that!