If you have a leak in the system it won't initially become noticable, its not until the level drops below the point at which there is not enough coolant to circulate that it will suddenly run hot.

The other cause for that is headgasket failure, this will pressurise the system and blow the coolant out of the overflow/filler cap, when it gets low enough it will overheat.

I only drove for a short period with the temperature in the red and stopped before the car really started billowing steam. However, once I filled the water bottle up, it ran OK, but it is definitely leaking water from the bottom of the engine.

Garage say it's a split hose, stopping the car before it really overheated probably saved the head gasket. Thank goodness as I have a far smaller bill to pay.