unless you have reason to believe that the theft was aided by the bike garage staff, i think unfortunately you'll only ever be covered by their (limited) insurance unless you get additional coverage
obviously there may have been laziness and/or generally poor security, but what would a receptionist have done anyway? this 'what if' case is unfortunately why lots of people prefer to be overinsured
i'm no expert, but i think that's going to be the case, unless you know who to badger about it