I have a water leak on my rover 400?

Its a Rover 400 Air condition Model T reg.

Its drinking water and the fan stays on after the engine is turned off and I can get no warm air from the Heater.

I know theres a leak but can see no water dripping whe n i have the engine running on the drive.

Does it have ot be a radiator leak or is there another possibility ?

I have tried a Radweld product but this has not helped.