Another winter and the leaking leon threads start again.
One word of warning, get it sorted sooner rather than later as your airbag sensors are under the seats and if they get wet then you have a much bigger
problem.
The few places they normally leak from have been mentioned.
1) Door seals (fixed with butyl strip which you can find on ebay)
2) Seal around plastic box under the wipers (scuttle pannel, driver side)
3) Pollen filter (passenger side of scuttle pannel) can become unseated and leak water.
There are plenty of threads if you use the forum search. It is also a sticky thread. Look in the FAQ section as well.