It is very common,
I'm sure there's a link on here somewhere. But here goes anyhows...
The seals around inner door card... The Ali panel under your door cards... The seals are dog sh.t being as when they were built they didn't take into account the great brittish summers/winters we have.
All you need to do I'd remove the door cards and unscrew the Ali panel. Run a bead of clear bathroom/silicone sealant round the bottom half of the actual door then replace the Ali panel. That should cure the leak. Need to do it on mine... Hope that helps...
Sent from my iPhone using Tapatalk