My detent spring broke about 3 months ago, too. It's recommended that you have the bike delivered by any safe means available rather than riding it further and risking worse damage from mashing through the gears. There is an outside possibility that the problem has to do with a loose bolt behind the shifter arm which sits inside the engine case. The factory improperly 'loctited' the bolt on some models and they can work their way loose. I believe that that particular problem was worked out by your model year, though.
Nevertheless, the clutch cover on the front of the engine will need to be removed in order to discern where the problem lies. Hopefully, it's just the spring, otherwise the engine has to be dropped and cracked open... an expensive proposition required to fix a part that isn't even broken and is entirely the fault of the factory.
More than likely it is just the spring. If it is, make certain that they fish around thoroughly and carefully with a magnet for any of the bits that may have fallen into the nooks and crannies of the front of the engine casing.
Triumph should be making good on this repair, as it is not uncommon, but, to my knowledge, they are disavowing the crappy spring and any problems it might cause when it breaks. If you have recently had your oil changed, have the dealer drain it before the repair and then add it back post-repair in order to avoid being scalped for a dealer-priced ($$$) oil change!