Once the cooling fan has kicked in, does the water stay about 90ºC or cool down to about the first mark on the temp guage again ???
If it cools down, I would suspect the thermostat. It may be that anyway as 10K (6 miles) is quite far to drive with a cold engine. Is there enough water in the system ??? Just asking as you say you've recently changed it.
Where is the leak ??? If it's from a join, you could tighten or replace the hose clip. If it's in the radiator itself, try using a "stop leak" thing like Radweld, which you add to the cooling system water and it seals small leaks.