turned out to be the plate that covers the rear end of the exhaust on my Mk1 Punto
Just wanted to write this as looked through forum for strange knocking noises and quite a few suggestions on here, but not this one, or rather I didin't notice this one.
The noise was so loud I thought car was going to break in two, but just needed bolt's replaced.
Worth considering if anyone else has similar noises.
Cheers

it is called a heat shield. it is one of the most common sources of noise on any car. usually all you need is bigger washers on the bolts to stop it rattling (it rots around and then drops off the original washers).