Well folks I duly jacked up the car, got under and took off the undertray.
Prodded and poked around, getting my hands around a lot of metal.
BUT! I couldnt find anything loose!
So I switch on the engine and yes still the rattle etc.
Then I realise its not down below its the big alloy shield on top of the manifold!

So I refit the undertray and drop the car down.
Start trying the bolts and they are solid, so cant see why the thing vibrates. So I decide to take it off in case its something else underneath.
Well it becomes obvious what the problem is as soon as I get the first (hex head 6mm) bolt out!
The ally shield is broke around the bolts! All four top bolts!
I then find there are 2 more nuts on studs under the heatshield (12mm) and eventually get those undone and can lift the great lump out!
Pics to follow!