Having read about the common water leaks, this thing has had:

Fake lead rooftape put over the the roof joins (its a highroof)

Front left drain gutter area welded and painted and siliconed inside (where its known to be bad)

Windscreen and seal replaced.

LH drive wiper arms holes blanked and sealed properly.

Rubber seal and washer around wiper arm spindles.

Seam sealer on the body panels wire wheeled and replaced as it failed and looked like side window leaks, when actually it was the body. The window frames were smeared in sika flex any way in the process (minibus version)

New aftermarket door seal rubber which helped keep the out side, outside.

But still...it leaks!

A puddle will form in the front left passenger foot well. I sat in the bus when it was raining to check and... nothing. Next day, it is a little wet. Seems to vary according to how heavy the rain is. I have a suspicion that it is pooling like a reservior somewhere, and when it reaches a certain level like a cistern, over it goes and wet floor.

Run out of ideas and resorces.

Maybe somebody knows where it could be coming from?