Sorry to hear you had problems mate.
One tip for freeing off exhaust pipes where they slide inside each other is to warm them up, usng a heta gun or blow torch and hit them failry hard right whre the two pipes slide inside each other. This helps to break apart the exhaust paste that is sealing them together.
I can understand why you want a garage to do it. Thers nothing worse than trying something and not being able to do it just because two bits of pipe won't come apart.
Do you have the standard centre section? If you do why not cut it out and replace it with a Remus centre pipe. This way you can gring or cut the remaining bit off the down pipe once you have cut it out.