So, is the flexi hose welded to the cat? Im unsure of what welding might need to take place?
Sorry if im being thick...
The exhaust is in about four sections between the engine and the rear bumper.
The cat is (I think) the second bit from the engine, and looks like a cylinder with the pipe coming out of both ends.
on the engine (?) end of the thicker cylinder will be a section of pipe which looks like it is woven from steel wire, rather than a solid piece of pipe.
This is the bit which rusts through.
Easiest way to tell where the exhaust is leaking from is to put the car on a ramp (or one side on a high kerb), start the engine, and block the tailpipe with a cloth - you'll see smoke come from where the hole is.
Once you've done that, simply unbolt the section of exhaust which is leaking, take it to your local exhaust fabricator (or welder), and have him patch the hole.
Should cost you around 20 quid + possibly a new piece of flexible pipe (maybe a tenner).
Bolt it back on, jobs a gud 'un