Typical dealer rubbish.... Find yourself a decent transmission specialist and take it there to be looked at.
I would trust a word they say, if its leaking its something to worry about, if its a tiny sweat then its not the end of the world.
Something else to note, the seals if they have failed dont just fail for no reason, normal cause is a worn bearing causing the shaft to move out of centre and wearing the seal out, depending on the leak location could be a plug loose, a driveshaft bearing, a prop shaft bearing or simply a contaminated and damaged seal.
Basically, take it to a transmission specialist and get a second opinion.
Or if youre near north london, pop it down to me and ill spend 10 minutes and take a look and let you know if theyre having you on or if it truly needs looking at.