This kept happening to mine and it was due to the pipes splitting close to the turbo around the jubillee clip. I know I should get some aero quip braided hoses but they are a tad costly. I have managed to stop this temporarily by taking the turbo beanie off as this seemed to focus the heat at the pont of breakage. I hear other members say that upgrading the rad cap moves the pressure elsewhere and I guess this became my least point of resistance.
As BBT stated though. once you switch off it then blows, not so bad in the driveway but once I went to a garage, turned my back, and on turning back noticed the steam pouring out of the bonnet and a crowd gathering.
That's bad enough, but having to fend off an enthusiastic fellow motorist who had grabbed the fire extinguisher and was about to set it off was more of a problem... guess he was also a St Johns Ambulance man in his spare time