Water leak!

Hi all I'm new to the land rover scene and loving every minute of it!! I recently bought a 2002 defender 90 Td5 it's been running fine until last week where the temp gauge would shoot up to the red and back down again whilst driving , so I checked the water level and there was nothing left so I filled it up almost 3litres. Then had to Travel 100 miles an it ran smoothly I thought great just low on water. Then thought I'd check it again just incase an the water had gone again. So I must have a leak somewhere, any ideas of where the problem could be before I start taking everything apart

All thoughts appreciated
Cheers

the most common place is the small pipe connection to the oil cooler cos it's hidden by the heat shield and the coolant evaporates due to heat, but it can be in many places....get a kit like this UV Dye & UV Torch Kit Leak Detection Dye For Cooling Antifreeze System RLD2 Ring | eBay , top up, put the dye in, drive it untill the level drops to the bottom of the tank then check with the torch in the dark and you'll find the leak cos the dye will be dried up there

Ok great stuff thanks for that I'll give it a go