1999 Other Rover Models coolant tank

Engine Cooling problem
1999 Other Rover Models Front Wheel Drive Manual 45000 miles

whilst driving I noticed steam escaping from the coolant tank cap, the level was low so I topped up with a water/anti-freeze mix. I then had a drive around the block(10 mins ) when I heard a sound similair to a kettle boiling. When I opened the bonnet brown water and steam was escaping from the cap again. Any suggestions as to what the problem may be, is this cheap to repair.
My car is a rover 200 1.6
Tanks