Nissan Repair Questions? Ask a Mechanic for Answers ASAP
This is more of a legal question than an automotive advice question and I would be more inclined to post something in the the legal section of the site.
The problem is you need to be able to prove that there wasn't a crack in the pipe prior to the garage completing the repair. They will no doubt say that the crack happened after you left the garage. If there was a crack in the pipe while at the garage then the vehicle would have had the EML on when it left the garage. The fact that you have done six miles and then the fault occured works in their favour because it was ok when it left the garage! Due to age and mileage there is no way to prove that this wouldn't have happened anyway.
As another example you could have an MOT test on your vehicle showing that the car is roadworthy. A bulb could blow on the way home making the car unroadworthy. Is this the fault of the MOT station? No, it is quite simply wear and tear and could have happened anytime.
I think you will find it hard to get the garage to claim responsibility for the problem when the EML wasn't on at the time it left the garage.