Engine overheating with temp gauge rising and blinking... Large amounts of coolant leaking...

I'm having overheating problems and coolant leaking issues in my 1985 Volkswagen 1.9L Transporter.. The temp gauge on the dashboard rises every time I start the engine.. only when I have been driving on the motorways does in stop blinking and the gauge goes down to normal temp... Once I stopped the car and shut the engine off, I noticed large amounts of coolant leaking from somewhere ( I couldn't locate the source ). It was at the back on the drivers side near the rear wheel.

Any ideas what is causing the overheating problems? Radiator, Thermostat, Radiator cap, Water Pump?

Any help would be appreciated.

Thanks,
Nick