It's worth taking the time to remove the rusted nuts yourself, it will save you a fair few quid. The three at the front pipe end are fairly easy to remove as they are just a straight through nut and bolt. Spray them with WD40 or something similar and leave them over night, then use whatever method you can to remove them, nut splitter, hammering a smaller socket on, grinder, hacksaw etc, then you can just replace them with the correct size nut and bolt.
The two at the other end are more tricky as they are bolts that are threaded into the flange, i find it easier to take off the back box and remove the mid pipe and cat together, then , when its off the car you can work at it with whatever you have got, they usually shear off anyway, but if it's off the car you can easily drill out the remaining bolts and either fit a straight through nut and bolt, or re thread the hole.
Just take your time and don't lose your temper with it, much better than shelling out over a hundred quid for someone else to do exactly what you can do with a bit of patience. I did mine the other day and it only took a few hours, and i was also welding the pipe back on the mid box as it had snapped for no apparent reason....
