Hi, just thought I'd add my experience here too.
I bought my Clio from a private seller in January this year. I should have picked up on the "deep clean" of the interior and the fact the outside looked like it had been sat in a field.
Anyway, it wasn't until a few days later when the car dried off and that the passenger footwell was still soaking wet that I realised the sunroof was leaking.... Badly!!
So.... I took it down to my mums partners shed (large barn) and we took the sunroof out and did the following....
Sanded and cleaned the roof of the car from any residue.
Cleaned out the corners where the water had been soaking into with a hair dryer and some tissue.
Removed the factory sealant from around the sunroof itself using a screwdriver (it was rotting away and clearly the cause) used a bit of sand paper and some white spirit.
Used clear sealant and was not shy with it either - all around the sunroof itself, then the roof. Carefully lined and placed the roof back on, allowing all the clear sealant to spread out.
Screwed everything back into position and done!
It was very fiddly with the screws and connecting the drainage pipes back up but it was worth it as throughout the snow and heavy rain over the past few months here in London, it has remained bone dry.
I was going to scrape the sealant off that soaked through the top, but it may weaken it so I've left it. Not overly noticeable and a small price to pay for a dry car. I may duck tape round it though to make it look tidy.
Not sure how much the sealant cost but it was a hell of a lot cheaper than taking it to a garage.
My car needs a lot of work and I have little to no pennies. I love it even though the electrics are funny and it's currently in the garage getting a new drive shaft and wheel bearing. I want to be able to do bits and pieces myself.