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Kev, I thought I had a minor heater core leak myself just a week ago. But all people I spoke with said you will have fog on the windows and a smell if that is the case.
It's not that difficult to look under the carpets, if you really want to, but most probably the leak is somewhere in the engine compartment.
The best method I think is to pressure test the system. It should be on a cold engine, I've found the leaks more easily spotted this way.
There are some places hard to look at where it may leak. Back of engine where the hard pipe from the water pump goes to a hose is one. Under inlet manifold is another. Don't rule out the cap for the reservoir. One possibility you don't want is a bad head gasket. Check for different coloration of the spark plugs.
In my case all clamps needed retightening after I changed all hoses.
When I put new clamps on new hoses I thought I tightened them real hard, but a couple of months later I had to retorque all clamps a number of turns.

HTH
Mikael Larsson