Not too familiar with the 1.4 engine myself, but on my 1.6 the only slight
problem I've encountered so far was difficulty starting the car when warm & the temperature gauge was not displaying the temperautre proberly, which turned out to be a failed control temperature sensor (and it's been reported that all
MK1 Leon's can suffer from this too).
Since having the waterpump & cambelt changed a couple of weeks ago, the temperature gauge
problem has returned and so it looks like that the thermostat needs changing now as well (both the control temperature sensor & the thermostat are either causes relating to the gauge
problem).
I was going to mention the dodgy door seal
problem that is the most common
problem with these cars, but I've noticed your Leon is on an 04 plate - if I am not mistaken and maybe someone could verify this, but Seat had actually corrected this fault on all cars from 2003 onwards hadn't they??
Liam