Firstly, don't take this seriously in the slightest. This is purely a bit of a laff with an old Omega that's going to be driven to the scrapyard soon due to a poorly auto box.
This is NOT, repeat NOT, anything you should do with a car that is to be relied upon!
Anyhow, here is how to repair rattling/blowing exhaust sections (if you don't give a toss about the car)..
1) Firstly, you need to identify which exhaust section is rattling. Get underneith and give the boxes a tap with a rubber mallet - the one on this Omega was clearly the driver side mid-section.
2) Being careful not to snap them off, remove the bolts holding the mid-sections to the cats, and drop the exhaust off the car.
3) As per the photo below, drill a couple of holes diagonally in the section, and cut out a neat as possible square using a jigsaw.

Now you need to identify what is clonking aroung inside the exhaust box. In this case, we didn't have to look hard for the foreign object:

Once the offending object is removed, you now need to cover the hole you have created. The first thing to hand was the lid off a family sized tin of quality street. Make sure you eat the sweets first.... and then cut out a template to cover the hole, as follows, and offer it up for size:

Now, you need to make the seal gas tight. So - using approx half a tin of "Gun Gum".. smear it generously all around the edges of the hole, as below:

Now, you need to place the quality street lid over the hole onto the gum, pressing it down firmly to make a gas tight seal. Then, once in position, you must drill small holes all around the edge of the tin, through into the exhaust section, approximately 2cm apart. Then, find some tight fitting screws, and then, using the most powerful electric drill you have with a screw-bit on it, tighten the screws. See illustration:

Now, using the other half of the Gun Gum, smear generously all around the edge of the lid you just screwed down, again, just to double ensure it's gas tight.

While the exhaust is off the vehicle, now is the time to repair any other little holes, using the same procedure:

Once completed, give the gun gum a while to harden to the touch, and it's time to mount the exhaust onto the vehicle.
Then you need to start the vehicle, let the exhaust get hot to help the gum harden, and you're done.
The end result? :

Now, take your £400 you would have spent at vauxhall, and give it to me!!
Repeat, this is a bit of fun, and is being posted for that reason!
It does NOT reflect good working practices and should never be done on a decent car! Unless you're stupid...