Welcome to the forum Bolin,
Any of these high street exhaust garages like the one you went to love to charge the Earth the second they see the badge with the serpant on,
Longlife, Powerflow etc though are all really good people who love working with steel
The tail pipe had corroded away on my Fiat Coupe 20V Turbo, it would have cost about £400 to get a replacement backbox / pipe from Fiat direct, or any other garages.
So I popped it into Longlife one Saturday morning, they removed the rear backbox, extended some new pipe from my centre section box to the rear end, stuck a nice shiny oval pipe on and away I went,
Not bad for only £90. And the sound was absolutely amazing.
You might consider ditching one of your own backboxes as well, it makes cars sound alot more fierce, especially Alfa's!!!
Good luck mate, let us know how you get on.