Hi Leon,
It is possible to carry out a number of tests to confirm the problem.
Most good mechanics will have a radiator pressure test unit (
http://www.mityvac.com/kits.html#universalcoolingsystemtestkit) - this device screws on to the top of the radiator in place of the cap and allows the mechanic to pressurize the cooling system to the desired level. By using this tool when the engine is cold, and again when the engine is hot, will enable the mechanic to determine the extent of the leak. If it is only a minor leak, that is, only when the engine is hot and the radiator pressure level is high, then its not a worry and the scheduled retighten will most likely correct it.
However if it is possible to create the leak when the engine is cold and pressure is applied to the cooling system then he should address the problem now as the head gasket will get worse very quickly.
If all appears OK using this test then it is possible that the cylinder compression pressure is forcing its way past the head gasket and over a water jacket to the outside of the engine. Again if this is the case tightening may correct the problem but it should be done straight away rather than waiting for the problem to get worse and the head gasket having to be replaced. There are also tools available for testing for this type of problem (cylinder leakage tester) which allow the mechanic to pressurize each cylinder using compressed air.
Hope some of this helps.
Cheers,
Craig.