They must be accepting some liability for this else they wouldn't offer any discount at all. The mechanic who inspected and 'repaired' it the first time you went in has been, from what you've described, at best careless and at worst negligent. You have driven away from a repair to your brakes, which have subsequently become unserviceable with terminal damage to the discs.
The 'closer inspection' that you mention should have been done in the first place. I would definitely pursue a full repair at their expense.
If they wont do it, get in touch with trading standard etc... to garner further opinion on it.