The chances are the window was not installed correctly in the first place.
The first thing to look for if the correct flashing kit has been installed (for slate,tile various profile for tiles) .
If it is the wrong one or has not been installed properly it's usually down to the head lap, not enough overlap(slate flashing) or side lap (tiles)
It could be possible that the apron at the top of the window (acts like a gutter) could be pierced due to a slipped slate.
It is possible to fix as velux do replacement parts.
I'd imagine it's very rare but if it's a fault with the actual window and frame then velux offer a 10 year warranty, but only enquire once you are sure that the window has been installed correctly.
If you don't have proof of purchase then there is a reference badge usually situated behind the handle, that verifys that it's within warranty period.

I Strongly doubt the possibility of the window leaking due to a build up of leaves, that would mean a lot of problems for velux if that was the case.
There windows are tested against all sorts of weather conditions and scenarios.
Is the window and flashing correct for the angle (pitch) of the roof? (wrong combination will result in leaks)
Your builder needs to check every possibility and scan all the way around the window and above to ridge height to determin no breakages above the window,
If there are then there's a chance water can work its way down the underlay or rafters and get in where the underlay has been cut open when window was installed.
Any of these suggestions should be easy to solve.
You mentioned you used a builder, I don't speak for all builders ( i beleive every trade should stick to there own) but you should really request a viewing off a reputable roofing company, plenty of experience counts for a lot.

I hope this is helpful.
Graham ( G.H. Roofing)

It sounds like it hasnt been fitted properly, they are quite easy to install.
Problems could be wrong flashing kit, front and back aprons not fitted correctly or the soakers/ side flashing.
Shouldnt be a big problem to rectify without changing the window, you just need a good roofer.

Hi catbluejaffa,

You can repair the seals on a velux...also can repair the roof flashing around it...

They normally only leak if they are a bit out of square, or not fitted properly.

hi the problem with your velux is down to the flashing kit easy repair

Hi, its probabley been fitted wrongly if it has leaked from day one. Yes it can be fixed and it shouldn,t of leaked in the first place. Have you had whoever fitted it back? I would suggest wrong or wrongly fitted flashing.

good luck

Hi
Velux windows should not leak, it is possible that a flashing has come adrift or it may be there is a problem with the actual fitting of the window in the first place, especially as its always leaked. I suggest you get the person who fitted it back to fix the problem.

get the installer to come back out and check if its their work or the window, as velux windows themselves dont usually leak! It must still be covered by the installers guarantee????

without being able to see the window and where it is leaking it is impossible to say if it can be sealed,
your best bet it post this in the roofing section of the site a get a local expert round to advize on options and costs.

merry christmas alex