take it to some where like kwik fit, i got my hks removed and my toda put on for £20.
QFT.
If you make sure the cash goes in the mechanics pocket, it shouldn't be more than 20 notes.
Swapping a manifold over isn't an especially quick job, especially if you need to swap the ARB at the same time to make sure the manifold doesn't knock. I can't see how any garage would make any kind of profit offering to swap a manifold and cat back for £20, time wise it's just too long. Cat back swapping for £20? Yeah that'll happen, but the manifold is a good hour alone.
I wouldn't trust kwik fit with a manifold, but a cat-back should be fine.