Topping up with water is fine. Especially in this weather.
When I know where it all went I was planning on flushing the system, replacing the rad and using coolant, just to update gone to it today and there is no water in the bottle, haven't topped it up yet to see how much it has lost.
The water fairies have drunk it all.
Its leaking from the bottom right corner of your radiator i bet.... Take a look for wet patches with a tourch.