Well, found where it was coming from......weird.....
The water is being forced out through a 2-wire loom that enters the heater box from the glovebox side. The wires are surrounded with a black plastic sheath (rather loosely fitting).
As previously stated, only happens when A/C is on. The higher the fan speed, the more water that drips out, so I figure that it is running down the inside of the box, entering the sheath, and being forced up it when the fan is on. Might see if I can push the wiring further into the box to prevent the water entering the end - worth a try anyway!
As I don't have my workshop manual with me, out of curiosity could someone tell me what these wires do. colours are yellow/green trace and white.