Hi. It's most likely to be water going under the scuttle panel at the bottom of the windscreen and down through the pollen filter into the car.
It can be fixed by removing the scuttle panel and cutting a sheet of thin plastic to fit up under the windscreen and down onto where the rubber strip goes along the front of the scuttle panel. It's also worth taking the pollen filter out, it's probably soaking anyway, undoing the two nuts which hold down the pollen filter housing, removing the housing, and sealing round the base where it fits onto the bodywork. Whilst you're in there it's also worth sealing round the plastic part on the drivers side of the bulkhead where the wiring goes through and finally cutting a square sheet of thick polythene and covering round the ECU to prevent it getting wet as well.