I had a blow from the front and was told by 2 different garages that my catalytic converter needed replacing at a cost of £400/£600. Decided to buy a second hand cat and heat shield and do it myself. When I got the cat off it was blowing from one side of the gasket and the cat was fine. Anyway i changed the cat and instead of buying a new gasket i cleaned up the second hand one (came with the heatshield) and fitted that. Blow was still there!
This all took about 6 hours lol but i can see how it's an hour's job for someone who knows what they are doing as you say. So I got a gasket for £14 from Euro car parts today and will pay someone else to do it. Hopefully will sort it.