If the switches appear ok then the bulb holder has to be a strong possibility, my bike is a '55' plate and used throughout the year. There is no seal on the rear light unit so all the road crap can easily get in. The fault on my bike started with a warning light indicating a blown rear bulb but the lights were still working. This was happening on and off for a couple of days and i then put in a new bulb in case it was a bad connection, it was ...holder corroded to F***
It is not an uncommom problem with the bike, from now on I will be giving the area a spray with WD40 every now and then to stop it happening again.
No doubt plenty more people on here will give you further advice/experiences.
Get it back to the dealer and let them deal with it.