Honda Jazz won't take water in the water fill cap?

I recently purchased a Honda jazz 2005 mode from a garage, top condition. I'm going on a long journey tomorrow and routinely like I would with any car I checked the water and oil levels. (cold engine). Water level was halfway between min and max so I attempted to top it up with coolant mix, however, as in poured the h20/coolant mixture into the water tank it just came straight back up as though the tank was full, although I can see it's only half full. Any ideas?

That is the way a coolant reservoir functions. Any coolant level above the max indicator could potentially be regurgitated.

You should not be adding coolant into a 2005 and from what you describe, its at perfect level, that water heats up and expands when the engine is hot, check the bottle when your engine is hot or after driving a few miles, it will be at the full mark, BEWARE never open the engine water cap when engine is HOT