why wonder, let an exhaust gas analyser decide, its the way a leak is spotted at MOT so it is what you should use as well.
patch up the hole using some putty, then drive it for a few days to make sure it has fully dried there is no movement on the repair that would prevent putty maintaining a seal. then take the car to an mot testing station and have an emisisons test carried out (usually free if you ask nicely and go when not busy, but never more than a fiver). if you pass there cant be a leak, or at least not one that is bad enough to worry about or fail an MOT.
i've seen very similar cracks on exactly the same place on several cars so i'm not 100% sure the crack you have found is the hole they were talking about (if there is even a hole)
the most important thing i can say at this point is to never ever go back to quickfit. they cost more, quote for unneccessary work, tell blatant lies assuming you have no knowledge, supply crap quality pattern parts that rot away in no time, hire unqualified kids on the cheap and dont check on the work they do, and worst of all they make you sit in a waiting room with a pushy sales prick who seems to think your tyres need replacing on all 7 wheels you apparently have without even looking at them.
