That smal lhole drains condensation down to an evaporating pan on top of the compressor.
The hole (and pipe below) often gets blocked with crud and water just collects.
You just need to poke down thru the hole and clear th etube down to the compressor.
This stuff has been useful to me in the past. You should be able to find it in a pound shop:
http://www.ebay.co.uk/itm/Net-Curtain-Wire-White-Window-Cord-Cable-With-FREE-HOOKS-EYES-Choose-Lengths-/331542750596
You MAY need to remove the outer plastic sheath of a short length.
XXX, March 2016
