Thanks for clarifying...yes its a manual gearbox. Undecided as to get complete system or just cat + centre box, i would be trying to fit myself so is it an easy job to attach rear box to new parts?
Have a look at the back box, see if it's getting ready for replacing. If it is, you might as well do it while you are under there. Have a good look at the joint, that's where your problems will start joining it to a new middle pipe. You will probably need a fitting kit for the old back box anyway, unless it's fairly recent.
I think the difficult bit is if the bolts are corroded. Also it's important not to over tighten them. There's a specified torque, typically my Haynes doesn't say what it is. The back box joint is supposed to flex, otherwise it can break off. Happened to me 
The bolts for the rear joint are shouldered, and you are safe to tighten them right up to the stop. If the joint is assembled correctly with the correct bolts and fire ring, the springs give the joint the flex required. Correct torque for these 8mm bolts is 22 Nm or 16 lbf.ft, tighten each side in stages so as not to displace the fire ring gasket.
The joints can be assembled incorrectly, you see this quite a lot - and if the bolts have been substituted for normal bolts, as sometimes happens when the old ones are snapped or cut off, it's never going to work properly.
