A reputable garage quoted £145 to replace the back box and middle section of (R reg) Puma exhaust.

However, on taking it in to be done they said they believed the Cat needed replacing and to replace the whole exhaust and cat would be £450.

However, another garage looked at it and said it is NOT the cat, but the engine is misfiring (on three) and it will cost either £20 or £100 to fix depending if it is the plug, plug lead or coil pack(?) which needs replacing!

Which sounds correct?