OH, well the rad should be perfectly ok as pace are quality.
What sort of quantity of water are you having to put in it, are we talking a few litres?
Does it still run fine and does it still run the same temperature, not overheating?
Mine used to loose a good couple of litres on a run, but if you got under the car to look for leaks there was no sign at all,i guess it was just acting like a kettle and boiling water out of the crack.
Other option is to have a coolant sysytem pressure check done, cheap and easy for a garage to do.