Yea liek said its usually the ariel.
Ive had both, on my old saxo my sunroof leaked in, to the extent that i ened up with puddles in the car about an inch deep!!
Garage charged me £60 to investigate exactly where the leek was coming from (which was the black trim round the glass on the roof) and then seal it up.
Now the saxo i have now the ariel leaks in lol luckely this is a little easier to fix, just pull out your interior light, you will then see a bolt witht he ariel wire attached, undo the bolt which will free the ariel, get some silicone, put a little bit on the ariel and refit.
Jobs a gudun!
Do the ariel first yourself, see if that fixes the problem, if nto take it to a garage to investigate the leek, BUT! shop around first, i got quoted up to £300 at first but then got it done for £60 liek said and that was from a VERY reputable garage